AFRO SAMURAI

After you have unlocked the double slice move (not sure if that is what it is called, you can check for it by pressing the back button and looking at your focus moves.) Wait until there are two enemies attacking you rather close by, press LT to go into focus mode then press and hold X until the shine gets to the end of the blade, then release, you should cut them both in half, very easy to get.

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 fight a lot of Ninja Ninjas at the same time. This is best because it has a lot of enemies that keep respawning so just wait untill there are a lot of him around you and then focus slice when you feel its appropriate.