I recently acquired this achievement by myself and thought I would put my own solution to it! First off, there are 12 maps required to completing this achievement:
-Three Kingdoms Casino
Next, make sure to set your enemy density to low and the difficulty setting to realistic!
The key to completing this achievement (especially if you don't want to die every 10 seconds) is yourteam. They can take plenty of shots and do not die as quickly as your character does! Make sure to keep them under cover though!
Your true friend in completing these maps though are Smoke Grenades!
- One smoke grenade thrown at your feet completely disguises you from enemies.
-Enemies are unable to see you through the smoke allowing both you and your team to safely take out quite a few guys.
-Thermal vision makes it easier to spot the bad guys and take them down.
-Don't worry about running low on smoke grenades because your team carries 10 smoke grenades, which is more than enough to make it through a map!
-Smoke Grenades can also be thrown near downed comrades, so that you don't get gunned down while healing your teammates.
Beware the shotgun!
-If you heat the distinct sound of the shotgun do your best not to get hit by it.
-One shot by the shotgun and your character is dead and nobody likes redoing a map.
Lastly, utilize your cover well and don't be afraid to send your guys forward. Often times I would find a way to keep my team in front of me, while I hid behind a wall and avoided enemy fire.
-Sometimes, you may have to "trigger" the enemy by running forward and getting them to shoot at your character.
-After this, you can simply run back to where you originally found cover and the enemies will get shot down by your team.
Good luck and try not to throw your controller at the wall too much!