this can be a pain to get, purely that the enemys are always jumping and moving around, and finding a clustered group isnt always easy, but the easiest thing to do is play on easy, and when you find a group of enemys just dance around them for awhile until you can line 5 up in a row and then bash em for the acheivement, but you may want to make a save once you get the suit or just before you try to bash the group, because you cant directly replay a mission, if you mess up youll have to start either the campaign over completely or revert to an older save further back then necessary.