the description for ghosts says that they can only be affected by Fire or Magic, what spells can actually be useful against ghosts?

Uhm... Banish Undead, maybe? Just off the top of my head

Uhm... Banish Undead, maybe? Just off the top of my head

That's the big one, but what else works?

I would say that any magic-based attack that deals damage and doesn't explicitly say that it doesn't work on monsters OR obviously does something to the physical properties of the target (like boiling it's blood). So for example Bear's claw and Stone storm.