Instead of being an idiot like me and trying to use the barrels for this achievement, go for unlocking the "Frag Thrower" skill (the sixth skill on the right). I completed this using just the initial unlock on the skill.
Throughout the game you'll run into situations where you're encountering numerous enemies at the same time (especially in "The Epilogue"). All you need to do is get three enemies within the "bubble" your frag drop shows and as long as all three of them die you should unlock this achievement.
Hope this guide helped! If you have any questions or concerns, lemme know!