Dishonored Definitive Edition

You [b]WILL have to Bend Time II.

And HIGHLY recommended you upgrade you Crossbow so it can reload faster

On mission 8 (Hound Pits Pub Mission) if you go to Piero's Workshop you will find Him and Solokov you will then need to go and get the blueprints for his Arc Pylon, when he asks you to "Put them all to sleep" or "Burn them all" choose "Put them all to sleep" this will make them unconscious find 4 guards and put them side by side on the ground, (Make sure you don't use a Tallboy, as I think they are protected in the head. When they are next to each other activate Bend Time II and shoot them in the head, it should pop after they are dead.

You can do this on different missions, I just suggest this one since it gives you more time to acquire Bend Time II

Just to add to the above solution
Rather then targeting city guards the following solution involves Weepers who have no armour and lower health. The below method also works with Time Bend 1 but Time Bend 2 is highly recommended. An upgraded crossbow is also recommended.

Load up Lady Boyles Party. from Mission Select. Start the level purchasing Time bend 1 and 2. Cross bridge and turn right past the Tallboy. You will soon come to a building filled with weepers. Make a save inside the building.

Walk into a hallway filled with weepers, walk amongst them so they all rise up and come toward you. Activate Time Stop and Switch to your bow. Aim for their Heads . Shoot them & move on, don't hesitate. You should be able to kill 5 within the allotted time.

Reload the save if it didnt work. Here is a quick 40 second clip of me doing it.

I found what I think is the fastest method using mission select (in case you missed the opportunities mentioned in the other guides and comments.

Select Mission 8: The Loyalists. You will need to have Bend Time II as well as a few each of bullets, crossbow bolts, and sleep darts. I also recommend having as many crossbow upgrades as possible, both to set up the necessary circumstances and to obtain the Achievement itself.

Ignore Cecilia the maid (she's not an enemy so as far as I know won't count as a target). Go up the stairs gr,ab the Abandoned Apartment Key on the hook next to the door on the right, unlock the door, and go through and down to the platform overlooking the street. There is a Tallboy stalking around and some guards chatting. Equip your pistol and blast the Tallboy until he falls. You could also use incendiary bolts or sticky grenades but the pistol is quick and simple. Now equip the sleep darts and start dropping guards. An alarm will go off and they will keep coming. If you run out of sleep darts, kill the rest of the guards until they stop coming. I recommend leaving a few more than four alive and unconscious, since the game has a tendency to disappear fallen foes. I didn't need to, but some players may find it helpful to stop time before taking out the Tallboy and guards. Just have a mana potion on hand in case you need a refill.

Once the scuffle is over, make sure you have four guards knocked out near each other. Make a quick save, use Bend Time, immediately switch to your crossbow bolts, and shoot each guard in the head before time resumes. Happy Achievement!

This can be done easily in conjunction with Tempest during Mission 08: The Loyalists. You want to activate the Pylon's sleep mode.

Activating the Pylon
What you want to do when during the mission is get the arc pylon blueprint from the second floor of the hound pits pub. The blueprint is at the end of the second floor hallway behind the white door. Once you enter the room with the white door, the blueprint is in a vent. Once you collected the blueprint, run upstairs to where corvo's chambers will be. You'll notice as you enter the top floor area a window that is open on the left-hand side. Run out the window and book it to where Callista would be located (Emily's Room/Lady's Tower). Once you are situated in [front of Callista's hideout, you will be able to see a ledge that leads into he second floor of Piero's workshop. Blink over there and talk to Piero. He will then give you instructions to activate the Pylon. Follow those instructions (whale oil tank goes up on the roof of Piero's workshop). Once the Pylon is powered up, talk to Piero and let him use the Pylon to MAKE EVERYONE ASLEEP.

After Pylon Activation
You will find unconscious guard bodies laying around. You can use Dark Vision to find them easier. Gather a group of six/four guards depending on whether you want Tempest or Hornet's Nest. Just get six bodies together if you want both. Once six bodies are piled up, make sure none have despawned cuz this game does that. Then manual save your game. I would recommend you use Bend Time rank 2 if you go for Hornet's Nest. Bend Time, whip our your di.. crossbow and spray those mfers. Once you get the Hornet's Nest, load your manual save, bend time and lob a grenade at the bodies. Cheevos popped.