Choose Select Level
Go to Chapter 6-1
Run forward to the right, drop down, and then run to the left. And then just start spamming the fire drawing. In about 30 minutes you should be done.
Best to attempt after the game is completed and all achievements won, as I only had 100 drawings left when I started this.
Here is a video I made to show you how I did the last 100 of my drawings after I had beaten the entire game and done the other achievements with gamerscore.