This is in the co-op mission Fire from the Sky
Thanks to blackstockc for manning the guns for me.
This can be done on any difficulty so for the sake of the achievement play it on easy.
**Just including a little sidebar for help if anyone is having difficulty actually flying the chopper. I know it has a steep learning curve and this is the best place to get accustomed to the flight controls vs. jumping in feet first in multiplayer. If you can wrap you head around the concept that your pitch (tilting your nose fwd and back) controls your movement to and fro instead of the throttle (which controls your alititude for chopper) that'll help alot. Aim the direction you want to go with your left stick and keep your rig balanced and level with the horizontal axis of the right stick. It'll take some practice but once you start thinking of flying it like a chopper instead of a plane or a car it'll seem a lot easier.
**Furthermore IGN has a pretty decent guide for flying here :
http://www.ign.com/wikis/battlefield-3/Flight_Controls_-_Hel...The biggest part about this is keeping the chopper steady and level so your gunner can take out the targets, as a pilot don't even worry about using your rockets. Play from the inside the cockpit view so you can monitor pitch, yaw, altitude etc. to keep it steady for your gunner.
RPG's and infantry shouldn't be too much to worry about as a gunner just mow them down. When the Anti-Aircraft guns show up this is what you need to focus all your attention on.
As a pilot keep at a decent distance for your gunner to launch guided missiles at them and take them out. The armoured vehicles are your secondary targets. Again just keep the bird steady for your gunner to launch guided missiles.
Eventually your Alpha and Bravo teams will make it to the extraction point and as long as you hadn't used RB for your fire extinguisher you'll net the achievement.