100% achievements can be roughly divided into three steps:

Step 1: Play through all 48 levels perfecting all of them
Step 2: Clean up the miscellaneous achievements
Step 3: Grind the remaining deaths

Achievements unlock only upon going back to the level select screen, which may cause a bunch to pop simultaneously if you play through a lot of levels without quitting.

All in all, this should only take 3 or 4 hours.

Get your first Critter into the Elevator.
Beat the Timebomb Hate Bot.
Beat the Buzzsaw Hate Bot.
Beat the Drill Hate Bot.
Dunk your first Critter in oil, and have fun doing it!
Beat the Flamethrower Hate Bot.
Beat the Zapper Hate Bot.
Beat the game.


These achievements cannot be missed and will be unlocked as long as you finish every level.

Get a perfect score on every level in Stage 1.
Get a perfect score on Split Second.
Get a perfect score on every level in the game.


Perfecting a level requires two things:

• Minimal critter deaths
  Basically requires you to make no mistakes. Some levels require you to sacrifice a critter or two to hit a button or something, and those won't count against your perfect rating.
  
• Flinging critters without the aiming line
  This can be circumvented entirely by just turning off the aiming line before releasing the critter. What I mean is:
  
  1. Right click a critter
     
  2. Turn on the aiming line (hotkey: a)
     
  3. Aim like usual
     
  4. Turn off the aiming line (hotkey: a)
     
  5. Release right click to fling the critter

With that knowledge in hand perfecting levels isn't difficult. There was maybe only one level which I had trouble with.

Reminder: You can use the aiming line as much as you'd like for all the achievements below.

Get blood on every solid object in a level.


Level 2-2
Fling all six critters into the top saw blade. With enough luck the split halves will spray blood on every wooden block as it bounces around. Getting blood on the smaller square block on the upper right seems to be the rarest.
You can fail the level and still unlock the achievement.


Save a Critter that has been char-broiled red.


Level 4-3
Play like you would by and explode the bomb blocks blocking the elevator with an oiled up critter.
Then drag a remaining critter into the flames until it's fairly red (just before it ignites) and fling it into the elevator.


Have every Critter (at least two) in the level die within a one second timespan.


Level 4-3
Send the oil drum left to where your critters are and ignite the oil drum with the nearby flames.

Level 4-4
Press the blue button by igniting the top row of barrels, but let all your critters fall to their death simultaneously.


Get 40 fires going simultaneously.


Level 4-4
I'm unsure if the flames on top alone are enough for the achievement, but you can just spread some oil along the left wall as well for more fires.
You can fail the level and still unlock the achievement.


Kill Critters by all available methods: falling, spikes, buzzsaws, explosions, fire, and electricity.

5-2 is the first level with electricity - the last of the 5 obstacles.


Beat Hang Time without the Critter leaving the screen.


Level 5-3
Easy one. Just don't launch the critter too high when hitting it with the spring.


Find two different ways to set off the Oil Drum in Despair Mail.


Level 5-7 (source)

Originally posted by horrorshow:

So you have to find 2 different ways:
1.) Let the oil drum float to the left inside a bubble. When it is in the right spot press the laser button by throwing the bubble-maker at it.
2.) Let the bubble-maker float to the top to the laser button (inside a bubble). Shortly before it hits the button throw the oil drum to the left in the right timing, so that it is set off in the right spot

Save more than three Critters in Six Enter - Three Leave.


Level 6-5 (source)

Originally posted by horrorshow:

... In that level you need to save 3 Critters for perfect and 4 for the last achievement. I made it with throwing the first 2 Critters directly in the first 2 holes & then let 2 fly with a bubble to the last hole + busting the bubble with the remaining Critters.

Save all Critters in Gone With The Wind in 45 seconds.


Level 6-6
Not much to explain, this one is all about going quick. What I did was:

1. critter #1 onto the red button
   
2. critter #2 onto the yellow button
   
3. save critter #1 by flinging it right
   
4. save critter #2 by dragging it right (make sure critter #3 and #4 don't fall to the lower step)
   
5. send critter #1, #3, and #4 into the elevator
   
6. save critter #2 by flinging it right and send it into the elevator

Reach 30 lifetime level restarts.
Get 100 lifetime Critter kills.
Get 250 lifetime Critter kills.
Get 500 lifetime Critter kills.


Self explanatory.
6-7 is by far the quickest for grinding deaths. The level has 9 critters that all start very close to a rotating saw blade.