AFRO SAMURAI

The easiest place to get this achievement (and the other achievements for mulitple slices) is towards the end of the game when you fight Ninja Ninja. As you a lot of Ninja Ninjas at the same time. This point of the game has the most and weakest amount of enemies so just wait untill there are a lot of him(4+ on screen)and then focus slice when you feel its appropriate.

You need to unlock the ability to slice 3 enemies at once, then just let 3 weaker enemies get close to you and use a Perfect Horizontal Slash to kill them all in one go.