As you can see here, I can be considered a trusted authority on Kill It With Fire achievements. But, of course, I didn't start out that way, so I made this guide to help others who might have been confused with some of the trickier ones.

This guide will give you an overview of every achievement thus far, but not in too much detail- that just takes the fun out of it, after all. As always, advice is encouraged, so if I got anything wrong make sure to leave a comment so I can fix it.
And as of 10/12/2021, this guide also includes the new Anniversary Edition achievements! Scroll to the bottom of the post if you want information about how to complete those.

Achievement Description: Kill a Jumping Spider in mid-air.


This achievement is easily obtained by accident. As a matter of fact, just swinging around any melee weapon nearby one of these spiders should do the trick after a bit.
If you're having trouble, try using an Energy Drink to slow down time and knock it right out of the air.

Achievement Description: Play a tune.


In the first stage, hit the piano with your clipboard multiple times. If you do it fast enough it actually begins to sound a little bit like a song.

Achievement Description: Destroy the gas station.


First, blow up the oil container behind the store.
Once you reach the end of the convenience store stage, you'll see the gas station portion. Use any ranged weapon (guns, the flamethrower, etc.) to attack the area, and it too will explode.

Achievement Description: Kill a spider after being webbed.


This one isn't too hard. You'll need Cheese Puffs or a stage after Garden of Evil to complete this, since you need to find a Web Spider. After it webs you, just try to hit it with the Clipboard or the Frying Pan.

Achievement Description: Kill every type of spider.


This one is pretty self-explanatory. You won't be able to get this achievement before you have the Paper Trail stage.

Achievement Description: Find every flavor.


This one is also pretty self-explanatory. You have to find every single flavor of Cheese Puffs, which also won't be possible until you've unlocked the Paper Trail stage.

Achievement Description: Execute Contingency Omega.


I won't spoil it for you, so I'll just say you have to beat the final stage to unlock this achievement.

Achievement Description: Kill an Exploding Spider with C4.


This one is tricky for one main reason: the C4. In the second stage (Close Encounters), you may have noticed an objective telling you to "find the secret room". Go into the study. On the bookcase, there is a blue book near the upper left. Click it to open the secret room, because, hey- all the best secret rooms are behind bookcases, right? If you can't find the book, just try every one.
Inside the room you'll find C4. Now go to the convenience store stage, throw it all over the place, and press R to switch to the remote. You're probably just one click away from getting the achievement.

Achievement Description: Kill a spider by dropping an anvil on it.


It may seem tricky in principle, but this achievement is easy with the right setup. You'll find the anvil in Barnstormer, near the back of the stage in an open-fronted shed.
Make sure you have Balloons and Cheese Puffs (any flavor). Attach two or three balloons to the anvil, so it's hovering right over the ground, and then scatter Cheese Puffs underneath it.
Wait for a spider to take the bait, and pop the balloons with your weapon of choice.

Achievement Description: Shoot a spider with the Revolver from a long distance.


This achievement is harder than it sounds (at least, it was for me). First, you'll want to find a wide, open space in any stage. Make sure to bring Energy Drinks, and, of course, a Revolver.
Brightly colored spiders are much easier to see from a distance, so if you can manage it, try to use an Exploding Spider, a Web Spider, or a Queen Spider (which is also bigger). Chug the energy drink, wait for the spider to run far enough that you can barely see it, and shoot.
If you get lucky, this isn't too hard, but it took me a couple tries to get it right.

Achievement Description: Decimate the hedge maze.


Just burn as much of the maze in Garden of Evil as you can, but be wary of how much ammunition you have left. If you don't have enough flame ammo, but you have the weedwhacker, you can use that instead- and it might even be faster than the traditional method.
You don't have to destroy the entire maze, just a significant portion.

Achievement Description: Sink a putt.


In the stage Contingency Omega, one of the first things you'll see is a very, very small golf course. Just pick up the ball and drop it in the hole.
If you're feeling more sportsmanlike, I guess you could probably whack it in there with a bullet or two.

Achievement Description: Find all equipment.


Find all the equipment, of course. Keep in mind that you'll need to complete a couple of Arachno-Gauntlets to get the last pieces of equipment, and you'll also have to decorate the Christmas tree in Consequences.
To my knowledge, there's equipment in every single stage except Contingency Omega.

Achievement Description: Burn the money in the safe.


Go to that same secret room in Close Encounters that houses the C4 (explanation of how to get inside is above, in "Live By The Sword..."). Open up the safe and torch the money inside with the Hairspray, the Flamethrower, or any other equipment you've got that can burn things.

Achievement Description: Destroy the printer using the RPG.


This one is pretty easy. All you need to do is find the RPG in the stage Paper Trail, and then blow up the printer near the start of that stage.
Note that acquiring the RPG in Paper Trail requires you to complete a couple of objectives first.

Achievement Description: Find every upgrade.


Find every upgrade in the game. Keep in mind that you'll need to complete some Arachno-Gauntlets first to find some of the upgrades in Consequences.
Click here for an excellent guide that can tell you all of their locations. Note that some upgrades are unlocked by decorating the Christmas tree in Consequences.

Achievement Description: Find every tracker upgrade.


All you need to do is find every tracker upgrade. As with the other types of upgrades, you need to complete a couple Arachno-Gauntlets to find some of the upgrades in Consequences.
Click here for an excellent guide that can tell you all of the tracker upgrade locations. As with the other upgrades, some tracker upgrades can only be unlocked by decorating the Christmas tree in Consequences.

Achievement Description: Knock every letter off Kill It With Fire HQ.


In the stage Consequences, shoot the letters spelling "Kill It With Fire Pest Control" with anything that will knock them off. The RPG is especially efficient, but any other firearm should work too.

Achievement Description: Transform a Spiderling hatched from a Queen into a Queen.


This one isn't very easy. Since spiders don't automatically grow up, you'll need to use the Royal Grape flavor of Cheese Puffs to transform the Spiderlings.
Kill the Queen Spider, and then start throwing cheese puffs everywhere.

Achievement Description: Complete a mission while in Tiny Mode.


Tiny Mode seems elusive at first- not in the options menu, or on the home screen for that matter. But all you need to do is nose around for an upgrade.
In the stage Consequences, there's an upgrade sitting in a drain that you can pick up. When you go back to the Upgrades menu, there should be a new upgrade called "Tiny Mode". If you don't already have enough points, refund them and rearrange so that Tiny Mode is activated.
Here's my personal strategy: Go to the first stage with Cargo Pants (infinite ammo, another endgame upgrade) activated, and rapid-fire the RPG all over the place. You should be done in under a minute.
Naturally, you can complete any stage in any way to get this achievement, so long as you have Tiny Mode active. Make sure to go to the Exit so the game knows you beat the stage.

Achievement Description: Find batteries.


Again, very self-explanatory. Some batteries are hiding in very elusive areas, so make sure to look very closely.
There's a tracker upgrade you can get under the van in Consequences that lets you track batteries and upgrades, but I haven't tested it extensively.
If you don't have that upgrade, click here for a guide detailing all the battery locations.

Achievement Description: Destroy EVERY dish.


This one is really tricky if you don't know where to find the last dish. In the Domestic Duties stage, all of the dishes but one are inside the kitchen- the last one is outside the laundry room's window, by a grill. The windows are breakable.
Having trouble deciding what the game defines as a dish? I had no idea either. Just whack everything that looks like it can break.

Achievement Description: 360 no-scope a spider with the Assault Rifle.


Basically, you need to turn 360 degrees, then shoot and kill a spider with the Assault Rifle. Keep in mind that you should probably start out facing the spider, or else the 360-degree turn requirement probably won't be met.
I advise that you lay out some Cheese Puffs to bait the spiders (the flavor doesn't matter too much) and stand over it. When spiders approach, turn all the way around and shoot them. Of course, you could do this by jumping first (what I did), but that could make it much harder.
Another useful tactic is using a Flashbang to stun the spider, making it easier to line up a shot.

Achievement Description: Face the music.


You need to complete every single Arachno-Gauntlet to get this achievement, including the one in Consequences. In case you didn't notice, that's a cockroach on a pedestal... but, not to worry, I won't spoil the ending cutscene for you.
Click here for a very helpful and comprehensive guide on how to complete every single Arachno-Gauntlet.

The most recent update for KIWF, the Anniversary Update, added some fancy new equipment for us to abuse arachnids with, along with a whole new stage and three achievements.
The below achievements all require you to enter (and at least partially complete) this new stage, which is called Down The Spider Hole.
For information about how to get all the new things this update offers, including the batteries, upgrades, and the secret community puzzle, check this guide.

Achievement Description: Betray a Spider Bro.


This achievement is by far the easiest of the new ones. There's a new flavor of Cheese Puffs in the stage Down The Spider Hole called Best Buds Blueberry. Lure some spiders using this flavor, and once they've eaten they turn into Bro Spiders. These little guys will actually attack other spiders for you, but that doesn't mean you can't still kill them.
All you need to do to get this achievement is kill one of your Bro Spiders. Poor thing.

Achievement Description: Unlock the shoe's true power.


Note: Use the guide linked above (in the "Anniversary Edition Achievements" section) to find all of the things listed below. I won't go over it in depth here.
This one requires a long and arduous process to complete, but it's not that tricky in the end. First, you have to collect all the planks for the treehouse ladder in the stage Down The Spider Hole. Once you're able to access the treehouse, find all of the small techy-looking pieces from around the map and place them in the basement.
After this comes the tricky part. You have to look through every stage and find a piece of the code you just built using the tech. Again, use the guide above to find these. Each person has a different code, so you can't rely entirely on a guide.
Once you've put in the right code, the door will open and you can get the shoe. As a weapon, it starts out pretty bad, but increases in power once you start getting more kills with it. Hit 200 kills, and not only will the shoe basically turn into a portable nuke, but you'll also earn yourself this achievement.

Achievement Description: Eliminate every blade of grass in the yard.


This one isn't too tricky if you've got a strategy. In the stage Down The Spider Hole, there's a whole lawn full of grass for you to take care of. Traditionally you'd use the mower, but... there's always a better way, and that way is called fire.
Using the Flamethrower to rid yourself of the grass is actually pretty easy, and probably even easier than using the mower. Just walk around the whole yard and torch it to the ground (but remember, using the Cargo Pants upgrade for maximum ammo is going to help out a lot).
Interestingly, it seems you don't need to mow every blade of grass, just most of them.

All in all, Kill It With Fire can give you a lot of entertainment past beating the core game. The achievements aren't too hard to complete with a little devotion, and I myself finished the game within only 12 hours or so of normal gameplay. And of course, the new Anniversary Update is even more incentive to keep playing, with all kinds of fun new gear to try out.

If you have any advice or revisions to recommend, please tell me down in the comments. This is still, of course, a work in progress.

Have fun, and keep on trying!