So I got this achievement completely by accident. When the level first starts, head to the left and find the part of the gate you can jump over. You'll find yourself behind a car with two mechanics discussing the ejector seat of the plane (an Opportunity you can follow for another achievement).
Once the mechanics have left, you'll notice a single guard over on the left in front of a guard house. You can sneak around behind him. Wait until the second, patrolling guard has turned and walked away and take out this guard. Drag his body back and throw it in the container to hide it (optionally, switch into his uniform as one possible disguise to get into the hangar). Importantly, don't touch his gun which will be on the ground near where you knocked him out.
When the patrolling guard returns, he should see the gun on the ground and/or wonder what happened to the other guard and get suspicious and search around a little bit. You can be long gone by this point, continuing with the mission, but eventually the patrolling guard will take the gun and move it into the hangar, unlocking the achievement.