Believe it or not, there actually isn't a catch to this achievement. A 110 gamerscore achievement is simply just for killing 150 total enemies with any weapon, magic, or equipment. It doesn't have to just be with one of them, either. You could kill 50 with a sword, 50 with spells, and 50 with traps and you'll get this. If you don't have it by the end of the game then something is very wrong.