The Dishwasher Dead Samurai

You have to kill 13 guys of those that have two weapons as the one on the achievement picture, pressing B as the final hit. That way, you will impal them. These guys don´t show up until the later part of the game, so you´ll maybe prefer to do it on easy. On that setting, if you only use X, X, X with the chainsaw, you can do that combo three times and then impale them with B.

If you have finished the game and doesn´t know where to kill them, just try the last level. Once you reach the last shop, there is a way up with two doors, one of the left with two bosses with guns, and one on the right with many invisible ones, but all are this kind of enemy. A good tip with these is that using your magic will make them all at once visible.