Choose Select Level
Go to Chapter 2-4
Tomb of the Giants
Run forward to the right, draw the vine all the way to the right so that when you jump, it will fall towards you, and then jump on it and jump off on the other side. You will see 2 critters. Run into one, and run away immediately. It will explode. Run towards the other one, and run away again, also making it explode. Then jump in the red vines, and you will spawn right before you draw the green vine. Rinse and repeat.
Helps you get progress towards drawing 2000 things as well, for the "On Sketchy Ground" challenge.
Also helps you get progress towards dying 300 times, for the "Clumsy Me!" challenge.
As I detail in my first run in the video, I die from the critter exploding. Then I reload and kill another one, and I do not get the challenge, even though at this point, I technically made 2 explode. But I had to actually made the 2nd one in the 2nd load explode, to get the challenge. That is because if you die from the explosion, it does not count. You need to kill critters without dying from the explosion.
Best to attempt after the game is completed and all achievements won, as I only had 2 critters left when I started this.
Here is a video I made to show you how I did the last 2 of my critters after I had beaten the entire game and done the other achievements with gamerscore.