VERY EASY SOLUTION!
Needs a partner and play it in coop.
Start the game, the first two training maps are perfect.
You got 10 ammo crates with you, just shoot your main weapon empty and let your teammate give you a ammo box and repeat.
120x 9=1080 bullets.
Keep one box for yourself and go to the next checkpoint where you get another 10 boxes.
Rinse and repeat.
We played it in the coop map "Afghan", i think there a 3 checkpoints =3240 bullets.
Play the level 3 times and then one more go and you got it.