The easiest way, I've found to obtain this achievement is during the Final Boss Fight (end of level 9).
After depleting your Adrenaline slots, one henchman comes directly at you, kill him in close combat, you'll get two gauges.
Then, 3 ennemies will spawn.
Wait maybe a little bit 'till they are all lined up and close, then fire away.
That should do it.
If it doesn't work right away, don't worry, this process of ennemy spawning will repeat itself quite a few times during the boss battle.