..you know, if we give a character with a powerful spell an unlimited amount of uses for the spell.. then make that person unreachable by planting corpses around them ( which would abound sooner or later anyway )...

Heck, we could even lock up a healer and a powerful spell person in one field so the healer could protect the spellcaster from dying...
I'd suggest Grandma Conquista with her Hellfishy.. I think. I didn't actually check who has the most powerful spells and lots of HP.

If you didn't get what I was saying, the scenario I thought of looks like this:
______
|SC
|CC

S= Spellcaster
C= Corpses of IMPENETRABLEstuff.
The spellcaster could only be hurt by spells.. and there aren't many of those in the enemy party, as it seems.