Dishonored Definitive Edition

All these other solutions are fine if you are having trouble actually getting this achievement, but a REAL simple way that requires no actual work to setup, is in the Lady Boyle's Last Party level (with high chaos), from the start of the level, go across the bridge (that you see the Tallboy kill those people on) and go to the right. It helps to have Dark Sight, but isn't needed.

Sneak into the last building on the right, and just after the first door where you enter to the stairs, there will be what looks like 4 Weepers all standing in a circle. Lean around the door jam, prime a grenade (by holding the button until the last two sides), and toss it in the center of the circle. There are actually more weepers than just the 4 here (hidden behind the others), so this achievement popped for me as soon as the grenade hit.

Oh, and there is one Weeper that will walk up the stairs as soon as you come in the building. Obviously don't get seem by him as all the others will run at you as well, but he wasn't included in the 6 when the bomb went off.

Hopefully this wasn't a fluke, and for those of you who don't want to go through a long and boring setup processes described in the other solutions, hopefully it will work for you as well. I did this on easy difficulty, so I don't know if that helped, but it shouldn't matter as it is a grenade, and they are weepers.

Extra chunky whale oil soup recipe

Start mission 8 - The Loyalists on easy

Refrain from murdering any guards, bend time is useful here.

Speed run to Anton and Piero in Piero’s lab, talk with them then go and find the blue print in Havelock’s room, bring it back and talk to them again. Choose to stun the guards and then fuel up the arc pylon and set it off at 1000 degrees for 2 seconds.

Gather up all the bodies (6+ for the protein) in a single layer pile in the open area in front of the lab, feel free to choke Anton and Piero out and add them in for added spice. Fill up 4 premium whale oil canisters at the station in Piero’s lab and sprinkle amongst the bodies. Once everything is set fire at one of the canisters to begin the blending process reducing everything to a fine soup.

Enjoy your soup and new achievement with Callista and a fine red wine.

Boyle Estate Mission

I simply waited in the buffet area until there were two largeish groups of people (that area really doesn't take very long for that to happen). I then just used Bend Time I on them, and then planted spring razors around the room. People got killed, I got the achievement, and then I simply reloaded my most recent save.

I believe Time Bend 2 is a requirement.

Level: Lady Boyle's Party
Requirements: Time Bend 2, Grenade.(When you load the level you are allowed to reinvest all of your runes)

Method:
At the start of the level, head across a bridge, go right past a Tallboy and head into a building. Make A Save near a large garbage bin.

Walk through the door into a hallway infested with weepers. Make sure they see you , even if they attack. Its important they gather around you. Blink your way to the back of the hallway - wait for them to approach you in a group.

Use Time Stop, Switch to grenades and cook it up so that the fuses is more than halfway burned. Place the grenade in the crowd of weepers. Head back to the wall and when Time Bend stops the explosion will kill the group.

Very Simple way to do it.
I made a pile of 5 unconscious enemys, put two Springrazors in this pile them a made another enemy to came after me and lure him to the pile of 5 bodies. The Springrazors was not even upgraded. Work it!!