Warlord beats Troll, Troll beats Elf, Elf beats Water Sprite, and basically everything else beats Enchanted Bunny.
You might try sliding in a little fold of paper between the wheel and the card to create more friction.
I've thought about that: cut card stock in the shape of a circle half the diameter of the spinner; use a hole punch to place a hole in the center of the circle; make one straight cut from the outer circumference to the hole; and slide between spinner and monster board.
I just have to think about how to keep the card stock from sliding out over time. Maybe just a piece of 1cm Scotch tape.