You are kind of missing the names. Here, I'll help.

Devan's Robot: Superbot.
The Witch: Wasn't her name Hazel or something? I don't remember. Anyway, she's not a boss.
Big Crab: Uterus.
Giant Eye: Bolly. It's actually a spark enemy in a metal machine, not just an eye.

Bilsy was in JJ2, not just HH98. I'm not really sure how Devan gained the ability to turn into a demon, but I suppose it has something to do with his being able to direct the inhabitants of the inferno and send them against his enemies.