For this achievement you'll need to unlock Water Trap with a Ki Crystal in the Powers menu under the Flux tab. Water Trap is activated by hitting
+
+
it will raise the enemies in front you off the ground for a few seconds. Upgrades can be bought to increase the stun time, area of affect, and grant additional buffs but aren't required.
At the very beginning of Chapter 17 you'll be running up a long stone pathway that is crawling with 30-40 skeletons and demons. As you make your way up this path get a large group of them in front of you and use Water Trap. You can use any weapon you like to kill the ones who are stunned and suspended in the air. I used the shotgun so I could focus on killing the stunned enemies and not blowing up the ones standing on the ground.
This level is also good for grinding a bunch of other achievements that require x amount of kills with certain weapons or conditions.