The Perfect Hunter achievement can be solo'd, but it's quite a bit easier to do with another person.
I did on the level St. Petersburg Banya, with a friend. My friend, or rather, comrade, got detected because he's about as stealthy as a brick being thrown through a window. I, however, remained undetected, and at the end got the achievement. In other words, if only your partner gets detected, you can still get the achievement, and vice versa! One can be a distraction, and the other can be a ninja.
It has been said that having reinforcements called in will void the achievement. I can't confirm this, but you should try to make sure the enemies don't call for reinforcements, just in case.
Note: People are reporting that the above method, as well as Maka91's method are no longer working. It looks as though Ubisoft has patched them. However, if you delete your cache(Go to dashboard > My Xbox > System Settings > Memory > Highlight Hard Drive > Press Y > Clear Cache) and play in offline mode, you should be able to use both methods again.
The achievement says nothing about retrying, but if you retry - For whatever reason - you cannot get the achievement. If you wish to start the map over, simply return to lobby(Deniable Ops screen) and start over.
Note: Col Biscuits believes that you must have a second account signed in as the second controller--it cannot just be a guest. So create a silver account, turn on the second controller, and sign in as the silver account you just created.
Here's a couple general tips:
* Bring fully upgraded Flashbang grenades. If you hit two enemies with a fully upgraded flashbang, you'll usually have enough time to melee kill both of them. Try it!
* Use your Sonar goggles often, and mark every enemy that you see. This will make it much easier to track their movements, and eliminate the need to constantly flip on Sonar.
Note: Use the upgraded Five-Seven pistol so you can mark four enemies at once!
Save your Mark & Executes for the following locations(In St. Petersburg)
Note: The enemy locations are semi-randomized. There are only a few variants that I've noticed, so for the most part, this should be quite accurate. For those of you who don't know, Hunter maps consist of 5 zones with 10 enemies each, naturally, you start in zone #1.
In Zone #2, you'll come to a large room with a bridge going across the middle, and bright lights on the right, and far left side of the room. Because of the lights, progressing without M&E is difficult. There's generally no more than three enemies in here, and they'll patrol in such a way that you'll be able to execute them without actually going into the room. Or, at least not far into the room.
Sometimes in Zone #2, there will be two enemies in a very large open room just past the room with the bridge, and to the left. This room is brightly lit, and the enemies patrol in such a way that makes killing them undetected quite difficult.
In the beginning of Zone #3, try to melee kill as many enemies as possible, because the hardest part of St. Petersburg is coming up, and you'll need executes. After you exit the mansion, you'll come to a large outdoor area, with a gazebo in the middle of it all. There are lights everywhere and usually about five or six enemies. You'll see some boxes in the alley that leads to the garden area, and near these boxes is a pipe leading up, and to the side of the garden. This'll give you a great vantage point over the entire area.
Couple more level tips:
In Zone #1, at the very beginning, there's a pipe leading up to several windows near the top of the mansion. Climbing this pipe will save you time, and usually set up an easy kill. Oh, and DO NOT SHOOT THE CHANDELIER!
In Zone #2, there are lots of pipes on the ceilings. Use these to get lots of easy Death From Above kills.
In Zone #4, the enemies are packed very tightly, and a lot of them are close enough so that hand to kills are quite difficult, and gunfire will attract a lot of attention. Be careful here!
Maka91 had a helpful tip to add:
You may also want to add the 2 controller method.
Person #1 gets the achievement. Controller 2, does the work. Fully upgrade his pistol and side-arm, as well 2 gadgets of your choice. For his suit, put all 3 upgrades towards armour, because he'll need it. (You can also switch out your main account weapons by putting them on the ground after killing an enemy.
Do the level with the second controller (not getting the achievement) and make sure to keep your main profile within a closer range, just in case you die. (Then your main will need to revive him).
Finish the level and the main account will get the achievement.