Shadow Warrior

I unlocked this in the fight directly following the first Beserker towards the end of Chapter 10.

Since you've just gained access to the Rocket Launcher shortly before this fight, this is probably the earliest place to get this achievement. I made some attempts beforehand with the Shotgun and Sticky Bombs with no success.

The Shaman will spawn lots of small enemies for you which you will want to land a Rocket into while they are clumped. Note that the rocket needs to hit them above the water. I also used a demon heart during the fight which may help out.

I'm fairly certain that the actual three enemies that died to give me the achievement I didn't shoot at all (and were not affected by the heart), but by that time I should have had the achievement 10x over so it could have just been late. Otherwise, you may have the same experience, meaning the achievement may be glitched. If you do, please comment.

This is simply the earliest opportunity. If missed here for some reason, this achievement will likely be unlocked when attempting "Slaughterhouse".

Hey everyone, just a quick video on how to get this achievement and a few others. Only really need the Rocket Launcher with extra damage, and the laser sight. Hope the video helps!