With eye protection I have cut the rock or rubble the zoanthid is attached to, then I broke the rock through with a screw driver twisting it in the cut, leaving the coral connecting the two pieces, and then I sliced the coral with a box cutter or razor. Cut with the dremel from the back side. If the rock is too thick then using a band saw to make cut to then break the rock would work. I wouldn't cut the animal with the dremel or saw.
Can you show a photograph of what you are trying to cut?