|Spam the RPG. Shoot the back of tanks for an easy kill, as well as use the air-fueled PRG for a 1 hit kill on anything really. You'll find the air-fueled on Chinese soldiers. This achievement can unlock randomly in co-op. My friend unlocked it while dropping out of an helicopter.|
Thanks to many people for pointing it out, you can also go behind the PMC and RPG the furniture.
If you don't unlock it by the time the game is over. Get a RPG with max ammo and save the game. Go out and blow stuff up with it and quit out. It will save progress towards the achievement but keep you at max ammo. Be sure not to pick anything up with the chopper, as it will auto save.