The easy way to get this is to use the head stomp, where you press LB while they're on the ground and you insta-kill them by stomping on their head. If you imported a character, you're likely to have this skill already. If you didn't import a character from the first game, it'll be a while to get this skill, as it is in the middle of the Combat skill tree. Shots to the head, slicing off the head and smashing it in with a blunt weapon still work towards this though, so you can always aim for the head with the appropriate weapon if you don't wanna wait long to get this the easy way. That said, even if you don't go for this achievement, you're likely to get it naturally as it's not that hard.