Hint: Summoned creature attack
If you can summon creatures to aid you they will auto-attack the enemy. This is especially useful when you can open a door without the enemy seeing you, Get a quick glimpse of the enemy and run back through the door. Then have the creatures summoned and they will attack the enemy. After they die, rest and repeat. You will regenerate your spells and hit points but the enemy will not. This is useful for the sphere battle in the lower level.