Ok mate, here is some tips:
note: I´m no pro or anything but here we go.
Hitbox: The Demons hitbox are only in theyr heads
Fluency: The game is runing on low fps, you need more animated frames
so it will look more "soft" and gets a better visual of it
Background: this is where I got a Lil confused, I didn´t get if the deamons where flying or runing, the sprites make them run not fly, and the robot jetpack indicates that they´re all flying. You should work a litle more on that.
I loved the robot art, looks cute, and the deamons either.. good job on that!