(This may or may not apply to only the Jelly Palace. It most likely applies to monster bug in general.)
So, it started off with a thorny jelly cube just walking out of boundaries, completely casual.
It was also quite normal for the monsters to sit there, let me kill them, and have them drop no loot. (But leave an invisible block and empty health bar.)
What had I learned?
A. Jelly Cubes are round, or cylinder shape. Not cubes. (I'm actually happy to know this for some reason.)
B. Sprite skills work like a machine gun if they had 0 cooldown, just hold down the key.