At the start of "The Covenant" mission, save your Spartan laser and use another gun until you get back to the beach later when you have to get in the Hornet. Fly the Hornet to a safe area around the Banshees, then get out and use your Spartan laser. Aim it at one of the Banshees and fire. If you destroy it you will get the achievement. If not, try again. Note: You will have five shots to destroy it.