DOOM + DOOM II

I couldn't figure out how I was killing bunches at the same time with the rocket launcher. Then I happened to hit 3 space marines with one shot and it popped.

I guess a "gib" is a human. Didn't know that.

I don't think the level matters as long as there are space marines and rockets. Obviously using the codes IDDQD and IDKFA helps a bunch (which is what I did), so I can't offer the best level to load up. I did level 1 of No Rest, but I don't know if there were rockets there.

Edit: I'm an idiot, I read the description wrong. Still, I didn't get it by blowing up groups until I hit 3 humans. Probably just a glitch in the Matrix.

Bottom line: hit a large group of enemies. I should have tried in the imp pit when trying the calamity blade.

Quick Guide for Console Players

Choose DOOM, select Episode: Thy Flesh Consumed (E4M9: Fear), and select difficulty "I'm Too Young To Die".

Immediately run forward to go through the first door to enter the first room, open the second door to encounter the four Zombiemen in the second room, weave around them to open up the next door to enter this large room, you'll have one Imp on your left and right side of the doorway.

Ignore them, the rocket launcher will be up a tiny staircase right in front of you, once you grab it, head back into the second room, and fire into the middle of the group of Zombiemen.

This should unlock the achievement.

This might take multiple tries, depending if the Zombiemen shoot each other or if the Imps decide to bully you, but this method worked super fast for me.