Gunfire Reborn

Simply put, when you're exploring a dungeon you'll spot distinct red or green colored barrels with either sparks or smoke emanating that will trigger effects at enemies that either gets them killed if they're close to them or afflicted with status damage effects if they wade into the ensuing blast area radius after you shoot the barrels (burn status for red barrels, corrosion status for green). Center your target reticle at the barrels when enemies are nearby before firing. I think grenades thrown toward barrels can blow them up too, not sure if that impacts progress to unlock this achievement positively or negatively, but you'll unlock this eventually as you play through and level up and unlock more and more characters, weapons, and scrolls.