2 Controller Method
From the main menu select the following; Multiplay > Custom Game > Halo 2 > Classic or Anniversary > Coagulation [Map]. Now simply connect your 2nd controller and log them in as a guest and once done, load up the game.
From the start, make your way to the centre of the map, and in the small gully, you'll find the Rocket Launcher right in the middle. Once you've grabbed this, take control of your 2nd player and have them go one of the nearest bases. Underneath the main structure or at the back outside of both bases, there should be a Banshee/Hornet.
Simply have them take control and now steel them towards the middle of the map. Once you're at decent height, level them out and grab your main controller - Take note that once the Banshee/Hornet are levelled out, they'll simply fly in a straight line or hover without you having to control it.
Once you're in control of your main player, press in
to zoom in on the vehicle. Now have your reticle over it and hold down
to lock on. Once the red arrows have centred around it, let go to fire. Once the rocket hits and blows it up, the achievement will unlock.