I am not sure if this is achievable but as it stands, NPCs tend to make these few mistakes which severely weaken their intended combat prowress.
1) Switching targets without having finished their full APRs.
When multiple players/summons attack the same target, the NPC usually switch among these players/summons without expending their full APR and sometimes, they even just stand still.
2) Spamming of spells that otherwise serve no purpose. For example, keep casting "Turn Undead" or "Meteor Swarm BUT running to player which invalidate this spell because Meteor Swarm ironically does not hit enemies within a small radious around the caster"
I've more examples but i believed that rather than improving their AB/AC or Spell Arsenals, their combat AI and responses may need a little look into.