To defeat the Guardians, first make sure that all of the enemies in the room are gone (not including the ones it will summon). Then, enchant a projectile weapon with the Strong Alignment, and blast away. It will not attack you directly; it can only use magic. Watch out -- each time it takes a hit, it will teleport to another location. After a couple shots (depending on how strong the enchantment spell was) it should go down.