Close to the end of the mission Yamantau, you'll face an armored enemy with a heavy machine gun.
Make sure to pick this up after defeating the enemy (who is easily killed with one or two molotovs). Just at the end of the level, you'll be leaving with your team, and you'll have infinite cannibals running towards the Aurora. Charge up your machine gun with
and fire with
at anything that moves.
You should have accumulated quite a few kills from this level, so you will probably unlock this achievement after killing roughly 20 of these. If not, reload your checkpoint and do it again, until the achievement unlocks.