Here's another approach for those not very keen on using the sniper method. It's important to know that
ONLY YOU need to stay undetected, your Guns for Hire don't matter.
Even if one of your AI buddies gets spotted as long as they don't see you the achievement is still going to pop. Simple strategy: Make sure you have the binocular perk and stay out of sight. Find some high ground just outside the outpost. Now just spot the enemies and send your buddy in to kill them.
If he dies simply summon another one.

Note: If they sound the alarm you did get spotted by someone!
This works on any outpost. Even though one would assume that The Judge is the perfect guy for this achievement I found that he often gets stuck shooting arrows that don't hit, especially if there are a lot of obstacles. Worked much better with Jerome but I imagine any AI buddy with melee takedowns will be the same.
I also had my Guns for Hire upgraded to the max which I recommend doing as well.
Good luck.