This challenge will unlock naturally by playing the game. If you've already finished the game and want to boost this I suggest travelling to the Altar of Mahthildis. In this area and on the way to the old monastery (right from the Altar) are the weakest enemies. You can use surprise strikes (sneak up on an enemy from behind / use Igniculus to stun them) to finish the battles faster. With a level 40+ character you should be able to beat these enemies with one hit. Once you've beaten all of them you can quit the game and reload to make them reappear or travel in another region. All in all it'll take ~30 minutes to unlock this challenge.