Cyclops

Cyclops is a demon found in Iris and the Giant.

"This cyclops is untouchable when hidden. On every other turn, it will throw its boulder at me."

Location
The Cyclops can be found on Floors 7 & 15 of Path of the Giant, and additionally Floors 2 & 4 on Nightmare difficulty.

In Path of the Ferryman they can be found on Floors 5, 10, 16, 23 & 24, and additionally Floor 22 on Nightmare difficulty.

Strategy
The Cyclops will first appear as a boulder and be invulnerable to your attacks. At the end of your turn it will pick up the boulder preparing to throw it. At this time it can be harmed, but if you don't kill it at this time then it will attack and return to hiding behind the boulder.