The wisps are incredibly easy, once you learn their trick.
They have an absorb shield. Put a fire totem out. Once the shields are gone and you stop seeing (+90 mana), unload with magic missile, fireball, magic missile... dead.
I've 100% them. ^_^ Last ability is incredibly useful short range teleport.
They have 1150 evasion... so don't even bother swinging a melee/ranged/magic staff weapon at them.
