There’s a rather easy way to do it without merchant tokens, and it involves completing an early game quest in the Damascus Gate Prison, for rescuing Ali Ibn Muhammad.
If you’ve done this quest, you may have used the underground access point opportunity. Use this to get into the Prison the easy way. Otherwise try via the rooftop as the courtyard has an open top. Just don’t drop down into it as you’ll attract attention, instead, climb down slowly as the patrolling guard in the courtyard is walking away from your position.
In the very center of the Prison, is a courtyard full of civilians and about 4 guards. Get into the crowds and try to whistle in one at a time, starting with the big guy walking around the courtyard.
Note that for a crowd kill to count, you must be inside the crowd or close enough to it for the kill counter to progress by 10%. You know you are in the crowd when your character and the crowd has a white aura around it.
When you whilstle them in, try to get them as close into the crowd as possible, you can only take 1-2 steps to the outer crowd circle before your white aura fades.
Once you dispatch all 4 guards, you may proceed into the 2 exits in the courtyard and try to lure more guards towards the courtyard with a combination of firecrackers and whistling. I believe you can easily lure 4 more guards in total.
Once done, just escape the area, DO NOT RELOAD SAVE (locks the counter if kills are performed on these enemies after a save reload). Instead, fast travel far away and give it a few minutes for enemies to respawn, then repeat it for the remaining kills. Do not load a previous save because the kills are tied to progression on the save, so reverting to a previous one sets it back to 0 or whatever you had before you entered the prison.
Note: If you are getting kills with a white aura and the counter is not progressing. Just hard close the game because Quick Resume is known for breaking achievement counters, you can also give your console the “hold power button for 10 seconds” treatment until it shuts down and unplug it from power, then leave for a minute or two before plugging back in.
