Start the level: The Storm. Now play untill you get to the space where the Scarab comes.quickly move to the bottem of the cranes where there are missile pods mounted. Look over the Horizon where you can see hornets and Banshees. Move your crosshair around that area untill it locks on to a banshee. Now fire a cupple of shots while its locked on. wait untill you see the banshee blow up. you should get the achievement. if not try it again. if the scarab comes the just let go of the pod and go destroy it, as it will block up your sights at the Banshees. once the scarab is gone try it again.
tip: If the scarab arives, take the lift up the crane. run to the end of the crane. wait untill the scarab us "close" to the crane then jump onto it. the you wont have to worry about taking out its legs.
OR you can Go into Multi-player and play the Map Valhalla. There is a banshee and a Spartan Lazer on this map so grab the Lazer (Splazer as some call it) and shoot the bird out of the sky.
Note: I suggest the first one over the second. As it can take a while to get the lazer, as because both teams rush for it.