[Semi Spoilers ahead]
This requires the Witch Hunt DLC.
Time needed: 5minutes.
Make a new Mage character on Witch Hunt, Max out your Magic Stat, Set all your stuff up however you want, Just make sure you get the Mana Clash ability.
Start the game, you will be talking to your dog, just skip all the dialog, Go into Flemeths Hut, talk to the girl, leave the house and you will be in a battle with a Gremlock and a Emmisary, Just cast Mana Clash on the Emmisary and you got your achievement.