So how do you make your robots essential so that they stop dying when you assign them to the arena?
Setessential 1001EED 1
Setessential 1011079 1

I found these commands on some message board or other, but can no longer find the source. The only downside is that player built automatons will still suffer limb loss that Ada and Codsworth are immune to. But don't worry, those lost limbs magically reappear after each arena fight.

While this guide is primarily aimed at end-game use, and thus have access to all automaton parts, I will take this moment to mention that all standard automaton parts are unlocked as soon as you finish the automaton DLC questline, and additional Nuka-World automaton parts are unlocked as soon as you kill the corresponding automaton in the Galactic Zone of that DLC.

Thus to have access to ALL automaton parts complete the following:

• Mechanical Menace[fallout.wikia.com]
  
• A New Threat[fallout.wikia.com]
  
• Headhunting[fallout.wikia.com]
  
• Restoring Order[fallout.wikia.com]
  
• Kill & Loot a Mr Frothy[fallout.wikia.com]
  
• Kill & Loot a Galactron[fallout.wikia.com]
  
• Kill & Loot a Nukatron[fallout.wikia.com]
  
• Kill & Loot a Space Sentry[fallout.wikia.com]
  
• Kill & Loot a Novatron[fallout.wikia.com]
  
• Kill & Loot N.I.R.A.[fallout.wikia.com] (preferably AFTER completing her quest)
  
• Loot the Project Cobalt schematics[fallout.wikia.com] (requires finishing the Nuka-World DLC)

TL:DR? You can skip straight to the “Bot Builds” section!

First, you should decide what role you want the robot to fulfill; do want a robotic provisioner, a constant companion, base defense, or what have you?

Next, do you want the robot to be primarily melee or ranged? Trying to mix the two tends to result in a robot that will only use one set of weapons at time and thus halves its firepower. Mixing weapons within a category is usually alright but matching melee weapons usually performs better than mismatched ones.

Once you have decided the robot's role and combat mode, your robot's arms & legs are basically predefined.

• If you want a melee robot – Assaultron legs are always best. The legs determine the melee robot's attack moveset and Assaultron legs just out perform all others. The Mr Handy thruster does come in close 2nd; but the thruster moveset depends on burst dps which can miss or partially miss, while the Assaultron legs provide consistent dps that just outclasses the thrusters' bursts. Plus the Assaultron legs provide more hp and armor slots!
  
• If you want a companion robot – the robo-brain treads give fantastic speed and maneuverability and I highly recommend them for any ranged companion bot.
  
• For cases where the robot will be little more than a stationary turret – the sentry legs give the robot increased HP and armor so that is what I use for ranged bots I leave on base defense

Likewise, your robot's arms will also be predefined by the bot's role and combat mode.

• If it will melee then always use the Assaultron arms – they have less hp and carry weight but increase melee damage.
  
• Otherwise stick to the sentry arms as they provide the best HP/armor/carry weight.

Sentry Torso?
The real choices come when you get to the head/torso; as long as you are not using the Mr Handy thruster for legs, you can always use a sentry torso to add shoulder mounted launchers to your robot. When I add them to a bot I always use one fat-man launcher and one standard cluster bomb launcher, the nuka-cola launcher is just outclassed by the fat-man and I don't find much use for the gas launcher. The shoulder mounts are a mixed blessing; unless you have the perk that makes you immune to companion damage, they can very easily kill you, and even when you do have the perk, your companion can still harm non-hostile NPCs with the shoulder launchers. Add to that the fact that shoulder launchers are terribly inaccurate and will cause your bot to enter a cooldown mode where it stops moving and attacking for ~5 seconds when the torso overheats. Whether you choose to use the sentry torso or not is up to you, but as I said it can be added to any robot build other than one that uses a thruster for legs.

Mr Handy Torso?
The Mr Handy torso allows you to make some very interesting builds, but it has one major drawback; you can not use any of the special head armor effects with it.
Additionally, as soon as you add arms to a Mr Handy torso, the thruster arms can only be equipped with melee weapons. So going with the Mr Handy thruster instead of the Assaultron legs, while having a worse melee moveset can give your bot extra bleed damage from all those thruster arm saws.
And for a ranged bot, the Mr Handy torso can actually be combined with Space Sentry legs (not normal Sentry legs, only the Nuka-World DLC ones) and still provide the 3x thruster arms. This might be a bug, but it allows you to stack up to 11x voltaic armor plates on one bot for a MASSIVE buff to energy damage. I put a Gatling Laser and a Heavy Flamer on this base defense bot and he murders anything foolish enough to attack.

Head Options
This is where you can make some truly ludicrously OP bots. Take note; the label about head armor having a chance to break is only if an enemy targets the bot's head and keeps hitting it there, this almost never happens so you will rarely ever see it break and even when it does it will be instantly repaired as soon as the bot leaves combat without requiring any extra parts or repairs from you.

• Instant kills – Golem/Doom Protectron Heads or Carnage/Reaper Sentry Heads all produce the same effect; the enemy just dies regardless of how much HP or armor it has, combine this with a fast hitting weapon like the Gatling Laser or Minigun and you basically enabled cheat mode. The downside is that your bot will be stealing all kill exp from you.
  
• Paralyze – The Basilisk Protectron head is also very cheat mode like. But take note that some enemies are immune to the perma-stun of this bot's head. I used a Gatling Laser paired with a Lightning Gun to spread the paralysis to as many foes as possible, and yes I used a sentry torso on that build just for more aoe. It is pretty ridiculous having all your enemies literally fall before you, but at least YOU get to kill some of them and get exp from it.
  
• Limb/Explosive Limb Damage – I tried out these helms on a dual-missle laucher robot and discovered that I much preferred a standard Cyclops Robo-Brain head over all of them. Why? Because the Robo-Brain head actively tries to back out of melee conflict, where as all the other heads do not mind sitting there in melee and missing horribly with all it's launchers (shoulder and hand) so my advice is to avoid this class of head armor.
  
• Space Monkey Sentry Head – While it acts as an extra voltaic armor plate, if you want maximum energy damage stick to the Mr Handy/Space Sentry hybrid detailed above
  
• Melee Head options – There are really only three other special head armor effects left to talk about and I would class them all as melee build options; Ahab's Sentry Helm, Spartan Robo-Brain Helm, and the Assaultron/Gorgon Laser Head. The Assaultron Laser Head is horribly inaccurate and fails against either of the other two in head to head match ups, but I still use it on my provisioners for the simple fact that it gives them the shortest engagement range possible so that my provisioners ignore most enemies while traveling between settlements and if one does happen to encounter an enemy it can't find a path to to melee, the bot has that one ranged attack to fall back on. Ahab outperforms Spartan when you can get multiple strikes with each melee blow eg when using Hammer Saws with Assaultron legs, it does however lack any form of ranged attack in that case so I'd use that combination on a melee companion robot.

I've mentioned a few good combinations above and there are many guides detailing this, but the gist of it is - there are actually very few GOOD weapons despite the long list of possible ones. Additionally, it is not recommended to use unstable weapons unless you are making a suicide bomber; eg one that breaks its weapons then runs in to self-detonate.

• For melee you want weapons that will hit multiple times with each melee strike; Hammer Saws do this when paired with Assaultron legs so that you get 3-6x hits with each attack, pair this with Ahab's helm and as much bleed plating as you can fit on the bot and that is A LOT of dots from each melee attack. Really there isn't any other contenders for melee weapons; even though the Stealth Blade might look useful, it really isn't.
  
• Twin Cryo-jets don't do much damage, but can give you some utility until you are able to use the Basilisk Helm, then they just get outclassed. Do not bother with the standard Flamer.
  
• The Gatling Laser outperforms the Minigun just because you are able to stack +energy damage from voltaic armor, and while twin Gatling Lasers work wonderfully I like to mix it up a bit and use something else that pairs well with it; eg the Heavy Flamer for damage or the Lightning Gun to spread Helm effect procs
  
• If you go Missile Launcher, go twin Missle Launcher. Reload speed is slow but it gives fantastic burst DPS, also provides nice Surface to Air capability if the BOS assaults the Castle with vertibirds.

That about covers everything, so I'll finish this off with a list of some bot builds to try out for yourself.

http://steamproxy.net/sharedfiles/filedetails/?id=839056197
Beginner's Mr Handy:
Mr Handy Torso w/ 3x Voltaic Armor
Tesla Coil Misc
2x Cryo-jets w/ 2x Voltaic Armor
3x Mr Handy Thruster Arms w/ 3x Voltaic Armor and Hand Saws (optional)
Actuated Thruster Armor for extra speed
- This bot will run up to enemies and freeze them, I do not think I've ever seen him use his hand saws which is why I list them as optional. He won't do much damage, but he'll provide utility by taking opponents out of the fight. He should be buildable from the start, so all you need are the mats to build him.
- Alternatively, you can equip 2x ranged weapons with Nuka-Mascot arms to allow a 3rd ranged weapon to be equipped on the Thurster Arms (technically a 4th ranged weapon can also be equipped to the Thruster Arms, but it is never actually used by the bot so better to keep the 2nd Hand Saw) This also allows you to equip another 2 eyes and eye armor, but the loss of the Voltaic Arm Armor is actually more of a DPS loss than the 3rd ranged weapon can make up for – still credit goes to Lilith for bringing this unique build to my attention

http://steamproxy.net/sharedfiles/filedetails/?id=839056101
Max Melee DPS Assaultron:
Sentry Head w/ Ahab's Helm
Assaultron Torso w/ 2x Serrated Plate
Tesla Coil Misc (or w/e since the coils will be weak without voltaic armor)
2x Assaultron Hammer Saws w/ 2x Serrated Plate
Assaultron legs w/ 2x Serrated Plate
- That is SEVEN different dots stacked SIX times EACH for a total of 42 dots with each melee attack this bot makes! He does however lack any form of ranged attack so this would be my companion robot. You can swap out poison for some of the bleeds in this build, but bleeds seem to bypass resistances while poison is subject to its own special resistance stat.
- Swap in a Gorgon Head Laser and Recon Sensors and you have a very effective provisioner build

http://steamproxy.net/sharedfiles/filedetails/?id=839056053
The Jezebel Variant:
Spartan Robo-Brain Head
Assaultron Torso w/ Hydraulic Front and Poison Back plates
Recon Sensors Misc
2x Assaultron Hammer Saws w/ 2x Hydraulic Armor (or Voltaic if the head actually works)
Assaultron legs w/ 2x Actuated Armor
- This was an attempt to make a “Damage per Hit” melee build with pieces that increase direct melee damage instead of stacking dots, there are some cases where this strategy works well eg against anything immune to bleeds like robots, but in the end I abandoned this line and just left Jezebel with it
– Since her head seems to lack the normal mesmetron attack I figured this was a good use for it and set her to be a provisioner.

http://steamproxy.net/sharedfiles/filedetails/?id=839065294
Procs-based Ranged Parts-bot:
Protectron or Sentry Head w/ your choice of insta-kill or paralyze head armor
(I recommend the Basilisk Galactron Space Head)
Sentry Torso w/ Shoulder Launchers for mass effect w/ 2x Voltaic Armor
Tesla Coil Misc
2x Assaultron/Sentry Gatling Lasers w/ 2x Voltaic Armor (optionally swap in one Lightning Gun for more aoe)
Robo-Brain Treads w/ Voltaic Armor
- The Robo-Brain treads give this bot great speed and maneuverability and since it is a proc build you really don't even need the voltaic armor or tesla coils; so you can customize the bot for carry weight or any misc mod you might need while still getting all your head armor procs with the fast firing Gatling Lasers or even Miniguns.
- Swap in a Cyclops Robo-Brain Head and dual Missle Launcher hands and you have yourself mobile artillery that will attempt to avoid melee whenever possible

http://steamproxy.net/sharedfiles/filedetails/?id=839056146
Mr Handy / Space Sentry Hybrid:
Mr Handy Torso w/ 3x Voltaic Armor
Eye Warmonger Serrated Plate
Tesla Coil Misc
2x Assaultron Gatling Lasers w/ 2x Voltaic Armor (optionally swap in 1x Heavy Flamer)
3x Mr Handy Thruster Arms w/ 3x Voltaic Armor and Hand Saws
Space Sentry legs w/ 3x Voltaic Armor
- Take note that this guy will charge and melee at times which is why we include the Assaultron arms, hand saws, and bleed eye plating. By having both the thruster arms AND sentry legs you get to stack a large amount of armor and +energy damage on this build. He isn't very maneuverable so I'd leave him on base defense, but he defeats everything else I throw against him in head to head match ups (other than insta-kill procs) and he does so very precisely so as to not endanger your settlers. Furthermore, this build is almost a direct upgrade from the Beginner's Mr Handy.

http://steamproxy.net/sharedfiles/filedetails/?id=839056244
The Jihadist Special:
Gloom Galactron Space Head (or Lament Sentry Head)
Hydraulic Sentry Torso & Arms (You don't want the infidels blowing you up before you can blow yourself up do you?!)
0x Shoulder Launchers (they take too long to break)
2x Unstable Explosive Miniguns
Actuated Robo-Brain Treads to get to his 42 virgin-bots ASAP!
- As long as you have made your Automatons essential or he is at least your companion, the self-detonation will NOT destroy this bot permanently, now if only we could get him to shout "allahu akbar!!!"
- Kidding aside this is actually a rather effective build assuming he comes back after detonating himself, you might even prefer this or the Parts-Bot to my melee provisioner since Robo-Brain treads are so freakin fast

If you have any constructive critismism or helpful comments - I welcome such feedback.
Or if you have any questions I'll do my best to help out.
Otherwise thank you for taking a look at my guide. :)
And remember to rate, like, and favorite this guide if you found it helpful :D