Thanks for the feedback! It's done via polygon area collision detection, but with a bit of a interior margin, you can control how easy the detection is on how far into the interior the internal polygon is. Each object and mouth shape have their own areas.
I think the first minute i didn't realize if i'm hitting the enemy or not, so maybe a sound there, or something. Other than that, in the given time, i think it's a solid game.
Now that i think about it, maybe in easy mode the detection could've been not so pixel perfect. Thanks for the feedback! Glad you managed to escape alive :D