Hi! Before you get into this guide, I just wanted to make sure you are aware I am not the best lambda wars player, and if you know better than me, great! But this guide is more or less for people who are starting out with lambda wars.
Also, I usually only play combine vs rebels, so if you're looking for guides on combine vs combine look elsewhere. That's all, I just wanted to get that out of the way before we started.

The rollout is an extremely important piece of every game. Your rollout can determine if you win or lose in certain situations. Rollouts are pretty simple, but here's how I usually like to do mine:
1. Build energy core.
2. While energy core is building, buy 1 more stalker and 3 loyal citizens.
3. When new stalker gets bought, send this stalker to the nearest scrap reserves; place down energy collectors on all scrap piles.
4. When citizens get bought, send them to the closest flags, each one going to a different flag.
5. Build a battery, or a couple batteries.
6. Hopefully you'll still have enough requisition to make a garrison, if you do, build a garrison.
7. Once the stalker finishes the energy producers, bring him back to home base, you'll be using him to build future buildings.
That basically wraps up how to efficiently prepare yourself for the game to come.
Feel completely free to make adjustments based on the current situation, if you feel like you need more stalkers or citizens, go right ahead, this is just how I do it.

Armory rushing is a variation of the standard rollout wherein you build the armory as soon as possible, for unlocking powerful tech (manned turrets, floor turrets) early on.
It goes a little something like this:
1. Energy core
2. Build 1 stalker 2 citizens
3. Battery
4. Garrison
5. Send second stalker to build a power node
6. Send citizens out to flags as they spawn
7. Buy another stalker
8. The moment you have enough resources to, buy an armory and have the third stalker and the first stalker build it.
9. Immediately research turrets the moment the armory finishes construction.

Earlygame is also important, how this goes will effect how much ground you hold.
Assuming you have atleast 1-2 flags captured, you should make it a priority to upgrade those flags; this makes them significantly harder to capture.
Get an armory ASAP, this will allow you to make use of all the utility this unit has to offer: the civil protection unit.
When you get the armory, research floor turrets.
Use your garrison to make pistol civil protection.
If you ever find time when your civil protection are not about to be in combat for a while, be sure to give them SMG.
These civil protection can be applied to nearly any situation, and are best used in locking down certain points.
Set up the turrets in areas you find your enemies commonly showing up in, and use it to defend certain flags.
These guys are going to be VERY useful to you in earlygame, as they are cheap and a great defensive option.
I'd set up a couple of turrets on your furthest out flag, this will deter enemy forces from trying to take it earlygame, leaving more of a window to upgrade it. It also provides 2 targets for the enemies to focus on, the turret and civil protection will both be doing damage.
Manhacks deployed by civil protection don't really have much defensive use, good for scouting and good for earlygame offense, so it can distract and thin numbers.
Outside of this, I'd also consider getting a mechanical factory (apologies if it's not called that, it's that thing that can make manhacks and stuff) for some offensive rushes, although this isn't required, it can save you a bit of requisition 'cause no civil protection will die from this.
Assuming all goes well, you should have a point defended and protected by turrets. Consider the covered area a temporary "safe area" as rebels won't be able to push through yet. Further units should be sent to provide backup.

Midgame is around when it starts to ramp up, things start heating up a little, and the demand gets higher for powerful units. It's beneficial to build quite a few batteries here, and upgrade your population cap. If you find yourself low on requisition, upgrade/capture more flags. If you find yourself low on energy, send some stalkers out to the 2nd closest scrap dump to set up more energy generators. Midgame should be focused on getting more requisition and getting synth factories. There's a button in the citadel that says "Combine faction abilities" in there, it shows a picture of a dropship. Hover your cursor over the dropship and check all of the requirements for it. You should try to build all of the buildings on this list as fast as you possibly can, unless there are some dire circumstances that require your immediate attention. Outside of all of this, I would maybe research AR2 around now, and I might send out a few AR2 overwatch soldiers to my furthest point just to be absolutely sure it doesn't get taken. I'd also use these AR2 overwatch soldiers as a kind of "spearhead" to repel against attacks to your flag, and to capture more flags.
If you manage to get a synth factory, start making yourself striders ASAP, if you have an abundance of resources, start making multiple synth factories ASAP.

You made it far! You should have a large abundance of resources, and you should be focusing more heavily on your striders. IF you have already met the requirements of the dropship in combine factions tab, send it out to your furthest point. At this point, if I have not already, I would completely max out my population cap, and I would focus on producing:
- Synth factories.
- Garrisons.
- Field units (AR2 overwatch soldiers suggested.)
- Striders.
Try to make as many striders and as many field units as possible. Upgrade the 15 overwatch soldiers deployed from the airwatch with AR2, then use them as another "spearhead" to go around and attack flags.
Make the rebels focus on those 15 overwatch soldiers. Make the rebels want to take them out. Wear down their offensive capabilities.
All the while, you are mass-producing striders and MORE overwatch soldiers back at base.
Make all of the striders and overwatch soldiers at home base group up into one gigantic group. This is your own army, this is what you will use to continue the fight to their home base.
Once you are satisfied with your army (I suggest 15 AR2 soldiers and 6 striders at the least)
retract whatever is left of your previous spearhead you got from the airwatch, combine these 2 groups to make an army that makes the 7 hour war jealous.
Make sure they all stay in a group and have them march towards the enemy base.
Rip and tear. Cross your fingers that you hold through and thwart the enemy.
Pay attention to the airwatch recharge. 15 more combine soldiers can turn an unsuccessful push into a successful one.
Hopefully you will win from this final push.

Ouch! That push didn't go to well. The enemies are probably dead set on returning the favor. Pour as many resources as you need to use into producing as many units as fast as possible. Striders, combine soldiers, whatever you can get out there that can fight is working in your favor. The enemy will hopefully be setting up an offensive, assumed that your push atleast did some damage. If your previous push, with an absolute crap ton of combine soldiers and striders, didn't do anything, I'd start saying your prayers. But, y'see, most often, 6< striders leave quite a bit of a mark, so you have time, but not much. Send stalkers to make more garrisons and synth factories. Hopefully, the enemy assault, when and if it comes, your forces outpower them. You can either use these units to attack or defend, completely up to you. Learn from your mistakes. Whenever your gonna go for a push, get some units queued up just incase this next one fails, as kind of a "safety net"

Stalkers can deploy a "defensive" structure capable of deploying headcrab canisters. These canisters are full of headcrabs, which are able of INSTA-KILLING most rebel infantry units AND instantly converting them into zombies.
I will go over the three types of headcrabs, what they are useful for, their specific abilities, and situations in which they would be useful.

STATS:
Headcrab:
Creature - Bite, Speed: 72
Dmg: 50, Firerate: 0.6 rps, Range: 512
60 HP

Converts rebel infantry into zombies instantly on hit.

Zombie:
Creature - Slash, Speed: 28
Dmg: 100, Firerate: 0.5 rps, Range: 55
250 HP

Fast Headcrab:
Creature - Slash, Speed: 256
Dmg: 40, Firerate: 0.8 rps, Range: 384
45 HP

Converts rebel infantry into fast zombies, if the fast headcrab inflicts the killing blow.

Fast Zombie:
Creature - Slash, Speed: 224
Dmg: 15, Firerate: 2.0 rps, Range: 50
150 HP

Poisonous Headcrab:
Creature - Bite, Speed: 32
Dmg: 150, Firerate: 0.8 rps, Range: 312
120 HP

Cannot convert anything via any means.

STRATEGICAL USES:

Headcrab:
Consistently your most reliable option. Jack of all trades type deal. Zombie form does massively better DPS and is much more tanky than its fast counterpart. This high zombie DPS also makes it very effective at handling swarms of low-health units, aswell as being pretty good at building DPS. Additionally, this headcrab instantaneously converts units, rather than needing to kill them outright like their fast counterpart. Practical and reliable in almost every situation.

Fast Headcrab:
Much less reliable than the headcrab, sacrificing the headcrab's general usefulness with a significant speed buff. Less health than the standard headcrab, harder to infect than the standard headcrab. Fast headcrab shines exclusively in how quickly they close the distance. If all the other headcrabs would take too long to reach the target, the fast headcrab will likely run in, deal a bit of damage, and die. Which is admittedly better than just dying. The zombie counterpart of this headcrab falls into the same niche.

Poisonous Headcrab:
Sadly, the only type of headcrab incapable of converting enemies into zombies (having poisonous zombies fighting would be so cool!) The poisonous headcrab makes up for this by having BY FAR the greatest DPS out of every headcrab or zombie that can come from headcrab canisters, being just enough to kill an ar2 rebel INSTANTLY! (assuming they don't have the armor upgrade, of course.)
These headcrabs are best used for building DPS, or attacking enemies that cannot be converted by the standard headcrab. Keep in mind they are very slow, but once they get into combat their health and DPS is immense.
You'll usually wanna use these canisters after you've gotten a good bit of standard headcrabs keeping enemies on lockdown, and are now just focusing on finishing off the buildings, because that's the specialty of the poison headcrabs.

ADDITIONAL NOTES:

Every canister, no matter which type of headcrab it contains, applies a brief stun and does a bit of damage to enemies that are near the initial impact site. Use this to your advantage, and send several canisters in quick succession to keep those enemies stunned long enough for headcrabs to deploy.
These canisters are most effectively used to thin numbers on a push before it reaches your base, pincer a push with your forces while their backline is being infected and attacked by headcrabs, or doing the final base offence without sacrificing units.
Do not use the empty canisters. They have a 2 second faster cooldown speed, and cost you a little bit less, but you could accomplish the same exact thing AND put headcrabs on the field if you used anything else.

In the following few sections, I'll go through all the most important units and buildings at your disposal. Using these units effectively and at the right times are paramount to your success.

The pinnacle of versatility and early-midgame domination. Civil protection give you access to a cheap damage dealer with two abilities that make them easily the most necessary unit on the field in early-mid that isn't the stalker. The floor turret ability, unlocked with the armory, and typically the sole reason for rushing armory. There is no limit to how many floor turrets one metrocop can deploy. The only limit comes in time, how many resources you have, and how many floor turrets your computer can handle before it crashes. This means that, if given enough time and resources, one metrocop can defend a point against literally anything the game can possibly throw at you. And, obviously, you can get more metrocops for more floor turrets faster, meaning that you can quickly lock down a point in turrets. Refer to the floor turret section on more specifics on the floor turret. The second ability metrocops have is manhack deployment. For every metrocop you have, they can have a manhack. These manhacks can either aid that specific metrocop in combat, or you can send them on spearheading missions. Get enough metrocops and you can basically instantly create waves of manhacks that can shred through any light units unfortunate enough to get in the way. Refer to the manhack section for more specifics on manhacks.

All in all, the metrocop has one weakness and one weakness only. Lategame scaling. The floor turrets eventually become inadequate to fight the hordes of rebels, and manned turrets become the favored alternative for their superior health and stopping power. Once the floor turrets fall out of favor, you can just use all your old leftover metrocops to man the manned turrets.

Never, ever, ever, EVER deploy a metrocop holding an SMG straight from the barracks. It is far more time efficient to deploy them as pistol metrocops and give them SMGs all at once.

The biggest reason to use metrocops. Floor turrets are paramount to earlygame defenses, allowing you to scale a small force of metrocops into a defense that can stop anything early and mid. These turrets do a deceptively high amount of damage, for a ludicrously cheap price. Floor turrets also mesh well with headcrab strategies, as they absolutely shred headcrabs and zombies, all without any unit intervention. Their only weakness is lategame, as they get outclassed by manned turrets as stopping power becomes more important.

The manned turret is an absurdly powerful behemoth of a defensive tool. They're like regular turrets on steroids, although slightly harder to get set up. Unlocks the moment the first armory is built.
One of these things can make an area completely untouchable to anything rebels can throw at you early to midgame. Three can make a defensive choke that even four hard AIs have a hard time getting past. But, of course, nothing's stopping you from placing a dozen or two. Whenever using these, always place a medical station nearby and keep a stalker on standby. The medical station can usually out-heal any flak the gunner takes, and the stalker can patch up the damage on the barricade itself to keep the gun shooting for longer. The only weakness this thing has is grenades, which you can counter with the energy walls. If you're trying to do anything difficult (like trying to win a 1v4 against 4 hard AIs), these things will be your best friend by offering ludicrously effective defense at a cheap cost.

The hunter is an incredibly versatile killing machine. It has a staggeringly high speed, allowing it to act as an offensive spearhead and quickly travel across the map to cover your weaknesses. Hunters are effective for stopping enemies who managed to sneak past your frontline, and excel in rapidly defending a point. For their speed and DPS, they have a really forgiving healthpool that gives them great sustain. This, paired with the fact that they (unlike their larger cousins, Striders), can heal at healing stations allowing you to continually reuse them. Hunters also serve the niche of being very effective bodyguards, allowing you to set one or two of them to follow a stalker and having that stalker be safe to travel unprotected territory to set up power generators or more defenses. Hunters are versatile, cost-effective, quick to produce, and almost always a viable option. The only weaknesses hunters really have are buildings and overwhelmingly large hordes of rebels.

The single tankiest unit you can deploy as a combine. Having a strider is basically the equivalent of having a walking wall with a gun and a dark energy cannon strapped to it. Obviously, striders excel in their tankiness. They're best used as a support for other units, taking the brunt of the damage and dishing out infantry-shredding blasts off cooldown. Striders have a mediocre DPS, considering how much you're investing into them, so they're mainly useful for their high healthpool and energy blasts.
Can never be hit by exploding barrels under normal circumstances, which makes them great basebreakers. Great at destroying buildings, if they have enough energy for a blast. All in all, not very versatile but when you find a situation they work in, they seem almost unfair to use.

Keep in mind that if you want to get the most out of your striders, you'll need to micro them. A strider using nothing but their regular gun and attack AI is a mediocre addition to your ranks at absolute best. You'll want to keep the automatic gun focused on units, as it does pitiful damage against buildings. Use the dark energy cannon to destroy buildings or particularly large groups of enemies.

Striders also can serve as great rangefinders for headcrab strategies. Their bulky healthpools keep them clearing fog of war for as long as possible, their dark energy cannon and immunity to explosive barrels make them perfect at getting rid of defenses, and to top it all off they are completely immune to the headcrabs and zombies themselves. One strider and enough headcrab canisters can often solo an entire base, with sufficient micro, as it is very difficult for anything to kill the strider before getting converted by headcrabs.

Manhacks basically hand you the earlygame on a silver platter. If you rush the building needed to build them, then use the metrocop (as you already should be) they SHRED early scouts and even large groups of partisans. Manhacks are most effective when used in large numbers and against anything classified as light or creature. Their incredibly high speed paired with cheap cost also allows them to be great scouters and rangefinders, if you don't have the time or resources for scanners.

Snipers might seem a little bit gimmicky at first, but if you use them right they can shut down entire pushes pretty much singlehandedly. Surprisingly fast building and surprisingly cheap for lategame, it's not that difficult to get your hands on a large number of them. The best use of the sniper is in bunkers or behind barricades. Hunkering down is the name of the game with snipers. A bunker can fit four snipers, which offer incredible one-tap potential against most threats on a decently fast cooldown. Pair this with a massive range and you can start to see their ideal use circumstances. They can thin out numbers in a choke before they reach manned turret range, then whatever makes it past gets mopped up by manned turrets. Not very good on the offensive, but if you insist using them to attack it's probably a good idea to bring a hunter or two aswell as a few metrocops to set up a forward sniping base. The only weakness of the sniper is that they require support to avoid being overwhelmed.

I'm going to go over some pretty interesting gimmicks. These are just some fun strategies to try if the regular meta bores you.

This strategy is only to be employed if you find your army not to be enough to take out the resulting defensive force and make it into their main base.
This strategy will only work effectively if your opponent is rebels, this will work alot less if your opponent is combine due to synth units and turrets completely countering any headcrabs.
This strategy, if preformed correctly and under favorable conditions, will completely remove the enemy's ability to fight back, as any units remaining will be infected and used against them, and they will camp the spawns of the enemies, meaning that you'll be able to freely march in, wipe out the remaining zombies and headcrabs, and take out the buildings without having to deal with any pesky enemies.
Firstly, in order for this to work, you need to have a moderately sized army on middle point. Y'see, a decently sized army on middle point will buy you enough time to get this working.
If you have an army that can fight off atleast a few attacks on mid, then you send in a stalker.
First of all, make the stalker build 2-3 healing stations as it makes your units much more resistant to attack, giving this strategy more time to work it's magic.
Once you've done that, build an energy core on mid.
Then, build a technical centre (the thing that produces manhacks and stuff) alongside 5-6 headcrab canister launchers.
Be sure key buildings are under the protection of either AR2-equipped overwatch or a good amount of turrets, as the enemy will jump at any chance they get to stop you from sending headcrabs.
Set up turrets facing the general area where you plan to drop headcrabs, as even a single turret can completely shut down quite a few headcrabs.
You're now ready to wear down your enemy.
Look at the map. You need to find a good position to start your infection.
Find areas where a unit could sit for a long period of time without being found or killed, that is also close to the enemy base.
You also want an area where the headcrabs can easily access the enemy's main scrap dump, this will also do a great deal to hinder future unit production.
Build a few scanners, and send 1 out to a suitable location where it wont die, but also far enough that it will be able to see the enemy.
Select every headcrab canister, and spam every single canister.
You want to try to get the headcrabs in such a formation that even if a grenade did explode, it wouldn't affect their numbers too severely. Basically, you want to spread out your headcrabs so they have to explicitly aim in the headcrab's direction, invalidating splash damage and making them harder to take out overall.
If their units survive this, keep sending in more and more waves. If they still keep surviving it, get more canister launchers.
Establish multiple scanners in multiple different locations, so they think they got what was causing it but in all actuality they didn't.
Try to spread out headcrabs in multiple different directions, and use fast headcrabs, too.
If you think that the enemies have lost the battle against the constant threat of headcrabs, send in a scanner and check their base.
Usually, the headcrabs will have already killed the majority of their buildings, and there'd be a couple of zombies here and there.
Deploy all of your units into their base.
Y'see, combine are WAY more resistant to headcrabs, and since there's no more coming, and they're all infront of you, a couple grenades alongside heavy AR2 fire will easily mop up what is left.
The most the enemy will be able to throw at you at this point is a couple AR2 rebels, which you will be able to kill almost instantly, causing you to completely destroy all ways of their unit production, before completely winning the game.
If you're still struggling with cleaning up the remaining headcrabs, you can send in a claw scanner instead of the regular scanner. Claw scanner moves slightly slower, but in exchange gives you the ability to deploy AOE mines. These mines are very easily able to wipe out a whole swarm of headcrabs, because if they are the only target the headcrabs will try to jump at it and continually fail because it's too high. This makes a nice big ol' glob of headcrabs right beneath the claw scanner, only for it to drop one mine and wipe them all out instantly.
If you're still somehow struggling with cleanup, just don't cleanup. Instead, place a bunch of turrets. Headcrabs are very easily killed by turrets, and this makes it so that you can just keep spamming canisters. The headcrabs can't attack you because you're surrounded by turrets. The headcrabs do a pretty good amount of DPS considering how many of them you'll have, so you can reliably just spam headcrab canisters at buildings and never even send in any units.
This strategy hinges hugely on your ability to hold an area close enough to their base to be within canister range. This is hard against more skilled opponents, because flamer spam requires a LOT of firepower to handle.

If you employ this strategy effectively, and on the right map, hunkering down well enough on the central point, this can win you 1v2s, and if you're good enough even a 1v4.

This strategy involves changing your rollout.
Basically, spend almost no time preparing for energy (i.e one battery, one scrap generator) and spend all of your time outside of that getting as many flags as you can and leveling them up as much as you can. Once you've got a good amount of requisition to play with, get a barracks, armory, then a Mechanical Factory.
Immediately research turrets, research nothing else, and start spamming barracks. Many, many barracks. Produce as many pistol metro cops as possible. NEVER get an SMG metrocop fresh off the barracks, because it takes 6 seconds longer to produce. You can make pistol metrocops faster, and then give all of them SMGs aferwards at the same exact total cost.
Now, upgrade your big glob of metrocops to SMG cops, send them to a good choke that's near the enemy base, and tell each and every one of those metrocops to set up a turret.
Yeah. And guess what?
Each and every one of those metrocops can deploy a manhack. An INFINITE NUMBER of manhacks. Those manhacks have to die before they can be redeployed, but still.
So now what?
Select every single metrocop, activate their manhack ability, select the metrocops and the manhacks, ctrl + left click the manhacks, and send the manhacks into the enemy base. Send them after defensive structures and unit producers.
Don't let the manhacks retreat, just keep them in the enemy base until they die because their lives don't matter. And guess what happens when those manhacks die?
More manhacks. You deploy an entire wave of fresh manhacks every 60 seconds. These manhacks can kill units, destroy buildings, even win the game for you. Doesn't even stop there.
You've got a glob of units that have guns. They can fight.
And those units are almost as easily replaceable as the manhacks you're sending in on a death mission.
Send a few of your metrocops in with manhacks following them. They'll cause massive chaos, and your armada of barracks will have them replaced in no time.
A lot of players are skilled enough to counter this, or the map isn't chokey enough to allow you to effectively shut them down with turrets. This strategy is inherently flawed, and is just for ♥♥♥♥♥ and giggles. If you still wanna win but are gonna lose if you just keep trying to spam metrocops, send a few stalkers out to the point you're holding down and set up headcrab canisters. Do that and you're essentially ensured victory, because those manhacks give you instant sightlines to deploy headcrabs through, while you work on setting up more effective sightline units, like claw scanners.

Same setup as the headcrab canister one, defending a central point and forming a base around it, but this time with claw scanners instead of headcrabs. Claw scanners have a surprisingly cheap cost, aswell as a very quick build time all things considered. Spamming out some synth factories and forming an army of claw scanners is a lot easier than you'd expect. Then it's just a matter of grouping them up together, deploying all their mines synchronously. With 10< claw scanners you can expect to instakill pretty much anything if all the mines connect (including the HQ). It's pretty cheap for what it offers (each mine is 5 req, so basically ~50 req for a large AOE instakill) and the mines have an INCREDIBLY short cooldown. You can also use this for its intended purpose, being locking down chokes with mines to make crossing more dangerous, which makes your center point hold even stronger. A great part of this strategy is that it meshes well with other strategies. The instakill can be really helpful for taking down defenses to soften up the enemy base to headcrabs.

This section is dedicated to fixing whatever might be going wrong with your plans.
First of all, once you get resources in enough abundance that you will not be set back with big purchases, you should make 3-6 more garrisons.
Hear me out on this one. Dedicate 1-2 of your garrisons to keeping a steady supply of SMG civil protection, and dedicate whatever is left to keep pumping out more AR2 overwatch.
The reason for keeping a constant supply of SMG cops is that you should spam turrets all over the middlemost point, more specifically pointing in the direction(s) of any potential foes.
Overwatch are extremely useful for shutting down offensives with their grenades + flanking, and civil protection are extremely useful for defending.
Basically, in the earlygame you want to use civil protection as a way to place down what is essentially another unit helping them defend.
If you find yourself dedicating too much time and effort into the airwatch deployments, just skip it! If you get enough garrisons out you can replicate the effects of the airwatch using just the AR2 overwatch. The whole purpose of the airwatch is not to get more units, it's to get more units onto the field faster.
Shut down their supply lines.
Be cheeky, send your army of AR2 overwatch to one location, and send 3 in an offshoot for one of their backline points, or even their base entirely!
Worst case scenario, the overwatch dies, costing you a minor amount of resources and time.
Best case scenario, first time you take out a flag, and then every time they see a push, they'll send a couple units over to their point/base to make sure you don't do anything.
Don't invest in shotguns, shotguns are easily outshined by the AR2 because of their lack of range, and the fact that the AI is programmed to shoot targets as soon as they are in range. So basically, shotguns only deal effective damage at a range the AI was specifically programmed not to go to.

Because there's always that one unit the enemy spams relentlessly that you're struggling to deal with.
I'll cover some of the more annoying enemies in the rebel's arsenal.
Flamer - Headcrabs, a bunch of widely spread out turrets.
Headcrabs; Although it might seem like the flamer is easily capable of wiping them all out due to crowd control being his main job, the headcrabs actually counter his AOE attacks due to the fact that they jump and make him flick his aim away from the other headcrabs, not to mention the zombies.
Turrets; The short range of the flamer means that a good handful of turrets is easily enough to damage him from afar, and also have it take a long time to get from one turret to the next.
Rocket Launcher - Headcrabs, rollermines + overwatch.
Headcrabs; Same thing as flamer. AOE is negated by the headcrab jumping, zombies are point blank and will pick apart the rocket launcher before he can deal significant damage.
Rollermines + overwatch; The rollermines soak up the damage, move in unpredictable patterns, meanwhile the overwatch picks apart the distracted rocket launchers while they're still chewing on the rollermines.
Headcrabs - Striders + city scanner, turrets.
Striders + city scanner; Yes, I know, headcrabs aren't technically on the enemy team, but they're still annoying if not handled correctly. City scanners attract headcrabs and zombies into one area, all bunched up, and the strider shoots a dark energy bolt. The AOE instantly kills most headcrabs, and the strider's normal cannon will clean up the rest.
Turrets; Turrets are probably the most effective counter to headcrabs, as they don't sacrifice any units, shoot the headcrabs from a range, and they absolutely melt zombies before they can get anywhere near the turret. A single turret can completely shut off a flank from getting overwhelmed by even the largest hordes of headcrabs.
AR2 rebels - Turrets + overwatch, headcrabs, annihilation cannon.
Turrets + overwatch; The turret absorbs damage and deals additional damage, while the overwatch supports it. The rebels have to get through the turrets before they can attack the overwatch, and the turrets are commonly placed in flanking positions to pincer the rebels.
Headcrabs; Obviously, the headcrabs convert the rebels into zombies, not only forcing the rebels to lose a decent amount of requisition and scrap, but also having that turned against them. Quickly thins out numbers, and if it doesn't finish them off a quick overwatch flush will fix that.
Annihilation cannon; If you couldn't tell, it's alot easier to deal with them on the defense. Annihilation cannons are probably the most annoying thing to deal with in the entire game, and will quickly thin out large groups of rebels. Best used with overwatch support (To distract the enemy fire.)
Vortigaunts - Overwatch, turrets.
Overwatch; Best spaced out, the overwatch quickly handle the antlions before finishing off the vortigaunt, spamming tons of overwatch is not only viable, but is also cheap.
Turrets; Prevents antlions from getting close, and quickly wears on the vortigaunt's health. I recommend putting multiple "lines" of turrets to counter the antlions flying out of the turret range.

This is NOT the only way, there are PLENTY of other ways to play, this is just how I play.
If you choose to follow any of this advice, it isn't essential that you do ALL of the stuff suggested, just the stuff you can reach for, I suggest you adjust your playstyle to fit what's currently happening.

Some tips and tricks I found while squashing enemy like bug.
- If you are fighting rebels, pay CLOSE attention to their units. If they like using vortigaunts, make sure that you send your striders in with backup, as vortigaunts are a counter to striders.
- If you want to use headcrab canisters, put them behind cover but close to where you want to strike. Send in a rollermine that doesn't have the command to attack. The durability will give you plenty of time to launch your shells before it gets taken out.
- Striders are best used with other units. Striders are good on their own, but can be countered. Give the enemies lots of targets and use the strider to support friendly units by doing extra DPS and using dark energy shots to take out high priority/high health targets.
- ALWAYS use synths. Synths make the game so much easier for you, and it's alot more of a challenge when you can't send a unit with 1125 health with a death cannon mounted onto it.
- Lay off of the civil protection when you get to mid-late game, rely more heavily on overwatch, this is as the civil protection will prove less and less useful as time goes on, good for some defence and you still can use them, but it's best to use the overwatch.

I really hope this helps you on your journey to dominating as combine! Remember: keep spamming those striders!