Kill an officer and blow up the armory with one shot in 'First Victory' mission.

Note: Getting spotted by the enemy ruins this achievement. So if you are detected at any stage, reload the checkpoint.

1.
Go out the door, staying close to the bushes on the left side of the area. Go straight towards the gap in the low brick wall. The enemy should all be looking the other way, so while crouched, sneak through the gap in the wall and immediately turn to your left into the bushes and keep going until you see a building with three windows.

2.
Still crouched, go for one of those windows (I prefer the middle window). Climb through. Two Nazi's enter the area and patrol close to you, the one going up the staircase on the left, the other moving away from you down the hallway in front of you. Crouch behind cover until both are looking away and moving off.
Now follow the enemy down the hallway, sneaking up to him from behind. Take him out silently with your knife. Continue down the hallway until you get to the corner and another staircase. Stay on the ground floor.

3.
What you do next depends on your own experience. Some players immediately turn the corner and continue towards the door at the end. I found that one of the enemies often spots me if I do this, so I wait behind the table at the corner for him to come along and stop at the staircase, then sneak up to him as well and take him out stealthily from behind. It should now be safe to move down the second leg of the hallway.

4.
You come to the door at the end of the hallway. Opposite from you is a room with two Nazi's, one being the officer you have to kill for the achievement. To the right is a door leading outside with more enemies patrolling.
Without getting seen (watching the mini map helps), turn left and sneak into the armoury room.

5.
Go straight to the bundles of dynamite and pick them up.
You now have the opportunity to blow up the armoury and get the achievement. The reason why getting spotted ruins the achievement is because detecting you makes the officer leave the room next door, making him survive the blast. But now that no-one knows you're there and he is still in the next room, his fate is sealed.
At the new prompt point a little bit closer to the door, access the bomb's timer by holding down F.
Use the W (up) and S (down) keys to set the timer to 15 seconds (maximum).
Again use F to activate the bomb.

6.
Sneak back out the door, going back the same way you came.
Take note: You will (most probably) die in the resulting blast, but the achievement will unlock, which is the goal here.
You need to be far away enough from the blast for the officer to get hit first. I've found that if I'm too close to the bomb, the achievement doesn't unlock. So move as far down the hallway as you can before the time runs out. The officer dies. You die. The achievement unlocks.

Duration: 4:02