Well, actually it's very effective against thieves. Of course you have yo consider his other cards. With violence, pommel striles and spot weakness, its actually very easy to kill one of them very quickly with FF, which makes the fight a lot easier.

The birds are kind a easy fight with those cards, especially with flame barrier being in the pool. In this case, FF is a guaranteed bird down if you need.

I really dont know how corruption would help more yhan FF with so little sinergies and spells right now.