This 'chieve really isn't that hard to get. I did it in a cave outside Flotsam, on a quest for an elf woman. Place is stuffed with nekkers, perfect fodder.
Try to have the Quen sign active as much as possible; this deflects damage. Also, save often. If you rack up a few easy kills save that progress. If you reload the save after losing health I can confirm the game will still count your progress towards the achievement. That's how I got mine :) Good luck and happy hunting!