Dishonored

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.

Edit: As some people pointed out in the comments, it is easier to get this if you had killed your way to this point as the extra weepers show up when your chaos is up. If you show up to the spot with low chaos (like not killing anyone in the game), there may be less weepers there to kill. - Thanks Sparechange49 for finding the problem, and Guirec730 for figuring out the solution.

Achievable on the first mission, though some preperation has to been taken before.

When you start the game at the pub, go to Piero and buy as much tranquilizer darts as you can(10 is max at that time)

Then start your first mission as usual, though be aware not to kill anybody, make good use of your tranq darts.

Once you tranquilized 5 guards, stack them up as a pile and put some whaleoil tanks next to the pile. Lure a sixth to that pile and make him explode the Whaleoil tank.

Also nets you the "Manipulator" Achievement as well.

As a note, don't try to stack up 6 guards, as at least one of them will despawn during the process.

I've had several complaints that the guard won't make the tank explode, in that case just shot the tanks when the 6th guard is near, this won't net you the "Manipulator" achievement though

Good luck

Check out the video for further explanation.

Late game spoilers!

The video with commentary will explain it all.

Unconscious targets make for easy kills. In the second to last mission, you can activate the arc pylon for Piero to put all enemies in the area to sleep. Just pile them up, and grenade or whale oil tank them. If you haven't gotten Hornet's Nest (kill 4 guards in less than 1 second with the crossbow), this is a good time to get that too. You need Bend Time level 2 for Hornet's Nest, but not for Tempest.

In the earlier game, you can get this too, but you need sleep darts or good sneaking skills.

Another way of doing this is during Lady Boyle's Last Party, go crazy and kill all of the guards. Once they are dispatched, posses the guests (who are now stationary) until you have six of them in a circle. Now at this time you may want to activate slow/stop time, lob a grenade (or a spring razor) in the middle of the group and POP... goes the achievement.

Why so complicated? Just knock out 5 of Slackjaw's thugs in his destillery, put them on a stack and put 1 or 2 Spring Razor Mines inbetween them. After that just aggro a 6th thug and lure him towards the stack.

He will activate the Mines and they will kill him and the 5 knocked out guys on the floor instantly.

Here's what I did. I was on the last mission, 09 - The Light at the End. I used the platform in front of the wall of light to the right (the one without the turret) for my unconscious body stack. I was actually going for Manipulator, but got this as well (when my first attempt at Manipulator did not work).

In this area there are plenty of guards and several whale oil containers. There are at least two containers outside (by the arc pylon) and several inside (via blink and the drain in the wall - if you are going for "lights out", you will need to enter this area anyways).

Now get crafty and stack your five bodies (see the Manipulator achievement for more info on stacking). Heed the other advice to do this, I choked most my guys, but I stopped after 5. I made my stack in front of the stairs that lead down to the water. I chose to leave the one guard, down by the water (way to the right of the platform/wall). This is the guy I need for the explosion.

Save now. You can use this stack for this achievement and the Manipulator achievement.

For Tempest, I put the whale oil container on top of the stack and when the guard came up the stairs and was close to the stack I shot the container. Boom, six down in under one second.

For the Manipulator achievement I held the whale oil container, got the guard to chase me, and as he came up the stairs I stood on top of my body pile with whale oil container in hand. The guard shot me, whale oil blew up, and the achievement unlocked. I managed to survive without a problem.

Two notes: This is a good mission for this as it seems all guards carry guns. Also, I may have had an extra whale oil container or two on the stack, in addition to the one I was carrying. That's why it's a good idea to save before trying to get the guard to shoot at you.

Link to related achievement:

Here's another spot to try, but you'll need these things first:

Level 7 Flooded District
Game in High Chaos
Blink 2
Bend Time 2
Dark Vision 1
Pistol with a several rounds
3 Spring Razors (preferably with the expanded radius)

Where: Near the end of Level 7 at The Old Port District, just after retrieving the key for the Sewer Gate.

Once you open the gate, move forward a bit and quietly take out the 2 Weepers roaming the catwalks. When you reach the more open area at the end of the catwalks be sure to eliminate the River Krusts on your left and save the game...

At the end of the catwalk is a short staircase that leads down into a populated area. In Low Chaos these are just regular people, and you can walk past them, however, in High Chaos, they are all Weepers and there are about a dozen of them in the area.

Sneak down the stairs about halfway and look through the opening to the back wall on the right. Using Level 2 Blink, you should be able to find that there's a yellow metal corrugated roof that you can stand on just above the main walkway. When you get the climb arrows, Blink there.

Turn around and you should be in standing on a small platform with Weepers all around below you. Take out your pistol and fire into the air. There are usually about 5 Weepers in the immediate area and they will shamble toward you. You may have to fire once or twice more to keep them interested, but if you stand (not sneak) at the edge they will remain enthralled. They will spit on you, so keep an eye on your health.

Now the tricky part is that the 6th Weeper will come from the area on your left (if you're standing with the wall at your back). Generally about three appear up there, and they will need to navigate down the nearby ramp to get to the group beneath you. At least 2 of them seem to get stuck attempting it, so the wait can be a bit annoying, but eventually one will make its way down to your group. You may have to fire your pistol into the air a few more times to keep things moving...

At any rate, eventually there should be 6 gathered below you. Wait until the group is good and clustered under the platform you're standing on and cast Bend Time. Switch over to the Spring Razors and drop down. Now place the Spring Razors directly on the Weepers. Try to get 3 of them just to be sure (my attempts using just 2 seemed to always seemed to leave one of the 6 alive). The Weepers that you attach them to will fall down, but they are not dead.

Unfreeze time, the razors will spring and the achievement will pop...

Here's another video in case anyone needs one showing a real quick and easy wasy to get this achievement, be sure to put the guards to sleep rather than to fry them otherwise it wont work....obviously.

I did this and hornets nest at the same time. Unconscious targets for easy kills. In the second to last mission, you can activate the arc pylon for Piero to put all enemies in the area to sleep. I did 2 piles of enemies to avoid then despawning, add whaleoil tank and shoot tank to kaboom. Easy and fast.

I was able to achieve this while going for

After completing the game, load up Mission 8 - The Loyalists. What I did was speed through the level making it to Sokolov and Pietro (running past every enemy and NOT killing them). Grab the blueprint for Pietro and tell him to knock everyone out with the Pylon (NOT to kill them). Because you didn't kill anyone, there should be more than enough enemies to find and line up. Cast Bend Time, level 1 works so don't worry if you didn't upgrade it at all, whip out your crossbow and there you go. Two achievements.

Video-guide of Dishonored to get the achievement or trophy Tempest. For this, first me must make sure we have 10 tranquilizer darts and select mission 2. We must gather together 5 enemies with explosive barrels and lure a 6th one to blow up it and get rid of them. It doesn't matter if we die or not. we'll get the achievement anyway.

Had to dunwall Whiskey Distillery

knockout 4 thugs

Start lining up the thugs in a row or a pile

Now it's time to turn these friends into enemies
To do this get behind a thug and try to knock him out
make sure not to hold RB too long.
When he headbutts you run back to the unconscious thugs

run around the unconscious thugs until the enemies gets near you then hold a grenade

Great answers all but this is a fairly easy achievement. Several areas throughout the first missions have 6 or more guards in an area. All you need is Devouring Swarm and a grenade. Guards will rush to help each other when a swarm is cast. I got this just inside of Sokolov's house. Cast swarm serval times between guards until all the guards have arrived and toss a grenade in to the mix. I was also able to duplicate this outside the Golden Cat. Pretty much any area with higher vantage point works well.

Just got this unintentionally and fairly easy without a lot of prep work. It was during the beginning of the Lady Boyle mission I killed all the guards and tallboys roaming around the city part of the level. Then just doing some searching, I came across an apartment to the back right hand corner after you cross the bridge. There were a group of weepers just standing around in a circle, I cooked a grenade, lobbed it in and boom, achievement!

I got this achievement while going to for

I found it easy and got it on my second try without having to kill anyone but the weepers under the stairs in the building. You're going to have to do this in your high chaos playthrough as obviously need to upgrade your powers and you have to hunt for ruins. The video below was great in helping me get the achievement really quickly.

All credit for the video below to GamerPerfection

If you thumbs down please comment as to the reason why so that I can make changes to the solution so that it's good solution. Please feel to comment any suggestions you might have to improve the solution.

Cheerio folks

Mission 3: The Golden Cat (House of Pleasure?)

Just after you come out of Slackjaw's Distillery - after he gives you your first sidemission for him. Note: you DO need a grenade for this to work.

Come out of the distillery, and there are three of Slackjaw's thugs just hanging out. Turn right down the alley, and you'll be ambushed by three assassins.

Keep backing away until the thugs come to join the fight (you might take a couple of hits).

All six of them clump together to fight, so a cooked grenade in the middle of them all will do it.

I had to try it a few times, but it was really clear that this would be a great opportunity to get this achievement.

I'm sure there are plenty of opportunities throughout the game, but this was the first easy one I came across that didn't require setup.