Your mileage may vary, but here is the spot where I made my last stand for the "One Guy Army" achievement:










Why did it work for me?

• It's an enclosure with 2 entrances on the left and right sides, which means there are a lot less mounted guards to worry about. I strongly suggest you stick closer to the right side as it's furthest to the city gate.
  
• The bottom portion of the enclosure is impassable but you can be shot over it so beware.
  
• The spot I am standing on in the screenshot has crates and food that respawns. Not sure what the respawn timer is, probably a minute or two.
  
• By standing near the wall, you will spawn and piss off A LOT of guards inside the city and you will be just far enough that none or few will path their way to you. Remember, the achievement requires to maintain maximum wanted status for 5 minutes, you can either fight hordes of guards of try to hunker down in a defensible spot.
  
• Finally, Look at the bottom right of the screenshot, specifically the minimap, the blue dot is where you will regularly get a guard spawning to fight you. If you kill him fast and don't go too far to the left, it should be easy enough.

If your health is high enough, you can probably also go for the achievement to drop max wanted level without dying and not being on a quest. To do so, just ride South to the pimp a horse, just be warned that there will be a LOT of mounted guards at max level, including some with lances to knock your ass to the ground. Better use a fast horse!