Another easy solution is on firefight.
Load up Windward on easy, then go grab the rocket launcher and a grunts plasma pistol. Go outside in the middle ledge and wait for the banshee to come for you, as soon as it gets close hit it with an overcharge, then switch to your rockets and boom, achievement unlocked.
(If you miss the first time don't worry, another banshee will come along soon enough :)