This is just some quick notes on ways monster intelligence could be improved. These are really just quick thoughts without any real design behind them.
Better use of spells (if getting hit by fire damage, protection from fire would be good. If damaged, healing is good, if not damaged, not so get)
Monsters cooperating more (cast healing spells on friends, fighters to the front, not spell casters)
Avoid damage when possible. For example, if there is a bolt down the hallway, they should try to step out of it if possible. Doing this right is perhaps trickier - smart monsters would take the damage in order to attack a player instead of sitting back and being killed by range spells. But for stupid monsters (slime, mice), it would be reasonable to use things like wall of fire to hem them in and kill them.
Smarter targeting. There isn't much reason to shoot a bolt spell if the player isn't in a space that will get hit by it (same hold true for an arrow). But smart monsters might cast a few spells - in a 2 wide hallway, cast a bolt down one side, take a step, then the other, etc. Likewise, even if player isn't directly in line of sight of monster, if close enough to get hit by a fireball, it should cast it.
Better pathfinding - monsters are not very good about finding their way around anything but the simplest of obstacles, which makes it fairly easy for players to lose/trap monsters.