A really easy achievement that can be done in a custom private match on your own in less than half an hour.
Game mode: Co-op elimination
Game type: Helicopter hunt
Enemy density: high (21) be sure to turn off infantry
Game length: 60 minutes
Map doesn't really matter, just one that won't obscure your line of sight. The choppers will come in waves of 6 (1, 2, 3, 4, 5, and 6 = 21 choppers). By the 4th wave you get MI-24s with rocket pods mixed in with the MI-17s. Weapon of choice would be the M60 or the MK48 hip firing, aiming slows you down. Pick a chopper to shoot down, trail and follow it until it starts to smoke then move on to the next chopper. You will probably die more than a few times, so toggling the respawn timer might help a little (the choppers will tend to fly and hover over your spawn points).