It may be too late to implement something like this if they weren't already planning to do so, but my ideal solution would be enemy AI that is roleplaying rather than trying to win a video game. By that I mean packs of wild animals that behave like wild animals, goblins that act like goblins, experienced mercenaries that act like experienced mercenaries, etc.
If an experienced sellsword sees a wizard looking fellow muttering and gesturing in the back of the group, I have no problem with him being smart enough to focus on that target. A newborn gnoll on the otherhand shouldn't necessarily be that smart. The problem with all these concentration spells is that the one casting them is almost always being focused down by an AI that is trying to play a game optimally rather than trying to simulate whatever character or creature. If wizard-in-back-fighter-in-front provided the amount of security that you might reasonably expect it to, I don't think it would be such a big deal.
We had a discussion waaaaay back about using some combination of enemy Int, Wis, and maybe even creature type to determine tactics.
High Int & High Wis? Knows to focus the casters and/or squishy dps characters.
Low Int & High Wis? Doesn't know to attack a caster specifically, but might use tactics to gang up on weak-looking characters.
High Int & Low Wis? Might focus on a caster exclusively even if tactically suboptimal.
Low Int & Low Wis? Attacks nearest enemy.
A beast probably wouldn't know to attack a caster, but an aberration would.
Etc.
