Hey man, I just learned a way to do this myself...
It it takes some slight work, but is quite easy...
I'll tell ya how without altering anything else...
First, go into the Object editor
1. Abilities
2. Make custom ability with Immolation as base ability
3. Go into Buffs/Effects, under (Night Elf - Buffs) select Immolation (NOT the one that has caster next to it)
4. Make a new buff with this as the base buff
5. On the new custom buff, select "Art - Target"
6. Edit the model, this is where you have to hunt... You need to go through them and select a model that you like... (if you move everything away from the left side of the world editor, you can see them in the window)
7. Go back to your custom ability and change "Stats - Buffs" to your custom buff.
8. Set the stats to your ability (if you want to change how much damage per second) or whatever... Change the name or anything else
9. Edit your unit to have the new ability
10. Test
This should work as have done it myself... If you have any further questions about it.. just ask
EDIT:
I would advise "purge" as the new model file...
Its under buffs