This achievement is very straightforward, and should only take about 2 minutes to unlock.
So firstly load up the final test. At the start, switch to hiding behind the car opposite, select your coin and just toss it near the car you were just at. The guard at the barrier should now come over, and you can subdue him (he's only a poor guard, so no need to kill him!)
Change into his uniform, move his body behind the car. Pick up his gun, making sure you have it selected. The walk past the barrier and you should see two guards to your right, just outside the door. Walk towards them (but not too close) until you get the detection bar from one of them - at this point push down on the D pad to drop the gun, and walk back to the barrier. Now select your coin and toss it to land next to your dropped gun. After this just walk and stand next to the two workers on the left side.
If you have done it correctly, the guard will approach the coin, pick it up (plus the gun), and go inside the building. After 10-15 seconds of him going into the building, the achievement should unlock! Good luck!!