The Hand of Merlin

Casting spells costs mana, simply spend 200 mana over your playthroughs and this will eventually unlock.   Make sure you finish each fight with a maximum of 4 mana so if you are rewarded mana it doesn't go to waste.  You will eventually unlock this as you are cleaning up and shouldn't have to grind specifically for it.

If you can try and use the Merlin restoration spell each fight, so you can heal 5 health to count towards that trophy as well.