Greater Archetype Power means that it will be usable at will by anyone immune to con damage. I'd go with a subclause that they take damage equal to their character level.

You might also look to metamagic for their spells - they might give up a Con point for Widen + Half the spell's damage is now Hellfire, and ignores Fire resistance and immunity. Increase the DC by 1, increase the CL by 1, and so on, making it a separate payment for each, and allowing each one at absolute most twice or so.

Looking forward to the combat tactics.