Can be easily done with a coop partner. Who ever starts the firing range (by pressing the button) will unlock the achievement so make sure you switch turns. You’ll need 15 targets in 20 seconds.
Available after you reach Sanctuary and right after you help Marcus put out the fires in his store.
I’ve noticed sometimes that some targets show up at the last second making it impossible to complete so keep trying!! Focus on splitting the firing range in half and focus on your section. You’ll know your group did it cause you’ll see confetti and unlock a challenge too.