Here are few things that could work for ghosts (with the 3 starting characters)
- wait for them to target one of your character, reposition the character and target the ghost. The ghost will be on cooldown (since its attacking) and won't be able to escape.
- use kick, grapplin or portal to make another enemy attack them
- they fly and can sometimes stand on top of traps (spikes, pit, etc), if you remove the fly ability (turning them into a chicken for example) they will fall on the trap
- damaging potions
Their ice ability is pretty good but they have only two uses, so it won't last long.