See with that. That is competing I would have made as a spell like feature. Or bumped it's base AC with an enchanted robe.Depends. Do the casters have shield? +5 AC for a round as a reaction interrupt to turn a hit into a miss can be useful for a CR 9 mage.
Or do something like have an abujurer straight immune to MM.
But again. What about the 2nd level spell slots? Are you casting attack spells? Are you casting buffs with those? Are you upcasting shield for 3 more uses?
Those 3 options change the mage's power drastically.