Plugin Help Listen for Player-Caused fire

I'm working on a magic plugin that increases player's "level" by how many entities he/she kills using their spells. I've managed to workaround all the spells so that when entities get killed by the spell, it would register as the player's kill, instead of some other natural cause. Except for one.
I have a spell that fires a fireball at the player, but I don't know how to listen for the fire damage caused afterwards. How would I listen to the fire damage caused by a player's fireball, instead of some other normal fire damage?

 

Bump
Still haven't found a solution : (

 

Ok let me try to rephrase my problem:
I'm looking for a solution sort of like the death message, [player] was burnt to a crisp whilst fighting [player/mob]. How does it do that - tracking who dealt the last damage, and give that player the kill instead of normal fire?