Boom Show Achievement
Defeat 20 enemies with Rocket's Five Barrel Barrage.This ability unlocks automatically during Chapter 11. Rocket pulls out a massive amount of guns and shoots bullets and rockets in the general vicinity of where you were targeting (he covers quite a wide area). You'll need to kill 20 enemies cumulatively for this achievement.
Regular enemies will quite possibly be killed entirely from a usage of this ability, but it depends on how mobile they are and if they get caught in Rocket's hectic fire. It's definitely more effective to weaken them a bit first, just to be safe. Shortly after unlocking this ability you will be in combat with an infinite amount of slug-like enemies, giving you a very easy opportunity to grind out this achievement if you'd like. There are, however, many large combat encounters throughout the rest of the game to work on this achievement.
As with other combat achievements, you could grind this in a single combat encounter by using the ability and getting kills, then reloading your checkpoint to repeat until you unlock this achievement.