You just get Keldorn cast Dispell on the mage (if none, then try to do it with priests/mages, although Keldorn's dispell is cast much faster) and charge melee with pretty good THAC0 fighter. That will remove stoneskin etc from the mage, and fighter will disrupt all futher spellcasting. At that level no mage should have contingency on half-killed with another stoneskin, so no trouble outta them. And if they DO have it, then dispell it again!!
Also, do not forget casting imp. invis on the fighter that will charge, it makes the mage suck with direct-damaging spells
By the way, this works even on liches guarding body parts if Kangaxx...