It is possible to kill 3 enemies at the same time with one swing of the sword if they are standing around you. If not you can use the explosive barrels that are in the game. There are 2 good areas on level four with explosive barrels. One is during the arena battle where the floor is electrified and a little bit after that is the part where you can use the gattling gun. There are a couple of barrels that the enemies will go near and i got this achievement by shooting them with the gattling gun.