One of the first possibilities, you can get this and also a very easy level to do this on, is the first boss battle against Behemoth (Chapter One: Fresh Meat - Boss Battle Behemoth
Make sure you select "Take it Easy" difficulty and just abuse the alternative shots of the Russian Rifle (
button). Shoot him repeatedly and he will drop ammo as well. Don't stay put and walk around and this should be a walk in the park.
Just make sure you don't press
to drink a bottle of milk. Also don't bash your head at the big bottle at the beginning of the level.