The quickest and easiest way to get this is to load up whichever episode you want (I just used E1M1) and use the cheat code DNSTUFF which will give you all weapons/items with max ammo.
After this just run around and shoot every enemy you see with the shrinker and step on them. I got the achievement half way through E1M2 this way. Or you could find a couple of enemies and use quick save/load (F6/F9) and farm them if thats what you want to do.
Following this guide will also unlock the following achievement.