I got this achievement on Denizens-disturbing revelations.
If you choose the path at the top (LT), you will go with Carmine and walk along the top.
I had a boomer gun at this point as well, from the earlier mob we killed. First of all shoot the first pink fruit from the ceiling on the left. DO NOT SHOOT the second yet.
There is a pillar in the middle of the room, I put a mine on the left side of this, and then ran over to where the second fruit is, I faced the small wall to the left and placed a grenade at head height here. Then I placed one on the small vehicle thing as well.
When you shoot the second fruit, and run by the vehicle, the first wretch should land on the mine and get killed, the second you need to kill yourself. More then sweep out of the corner of the room, and scatter, a few will get killed by the wall mine, and then others by the one planted on the small wheeled vehicle.
May take you a couple of tries to understand the layout and get familiar with the points above, but once you have it sussed, should only take you 10 minutes. As long as you keep getting the first wretch at least, it will take you 10 tries max.