The achievement asks you to defeat over one-hundred enemies.
I earned this achievement as I progressed through my initial playthrough. It doesn't matter if you engage them in open-combat and kill them or if you sneak up behind them and perform stealth kills. Either way, you'll find enough monsters to slay and you will eventually defeat one-hundred monsters. I earned mine when I killed an enemy archer, who turned out to be the one-hundredth enemy that I killed. The achievement unlocked right after I slayed the beast.
Have fun and good luck!