Get a Celestial Master with 2 Astral to Paralyze the Flaming head. Then just use xbows.

If you don't have a Master with 2 Astral, have 1 become a Communion Master and use 2 others as slaves to cast it.

Put a Ring of Fire Resistance or a Burning Pearl on something with some high strength (Or whatever your best melee unit is) and have him go pound on the sitting duck and he won't get burned by the fire aura.