Inferno Dragons are the little red creatures with the forks. You need to kill 20 of them. A good place to grind them off really quick is Level 10 (Invaded) - Hot Dip. You can kill 3-4 easily with a single run-through.
If anyone can confirm that you have to finish the level for the kills to count, let me know and I will add that in. I unlocked this accidentally while going for Crowns.
EDIT: Thank you DRISSTY for pointing this out: "Level 16 Invaded is a much better spot if you're not worried about getting this achievement right away. I was able to kill 12 Inferno Dragons in 1 run through."