Get Even

My advice for not raising alarms is to sneak up on an enemy by crouching and approaching them from behind and taking them out with . The more guards that are killed, the less danger there will be of being found. If you are detected, reload from checkpoint and try again (yellow is safe, red is not).

For this one, the fastest route will be to walk into the warehouse and immediately take out the first guard from behind. Move along to the right and hide next to the crate. Use the corner gun to take out the guard on your left.

Walk towards where you killed the guard and there is an entrance on your right. Enter here and carefully kill the two guards with a headshot. Move forward and the exit is at the far end of the room. This will not kill all guards but it will save time.

If you want to be extra careful, after killing the second guard climb the nearest steps. Move forward towards the guard overlooking the warehouse, crouching the entire time. Wait for them to stop talking and then take the guard out. Sneak downstairs and take out the guard on your LEFT first, then go for the guard that was on your right.

When you walk into the exit, the achievement will pop as soon as you move past the trapped prisoner.