This mission is much more simple than what I make it out to be in this video. If following my guide is a bit too challenging, instead climb the ladder closest to you when the level starts. Then use anesthetic on the first guard on your left, take his clothes and drag him behind the crates. Grab your gear, get in the truck, then proceed to the warehouse elevator and follow my guide from there. The guard will wake up as the rest of the guide takes longer than the five minutes he will be knocked out. It won't seem to matter as you will still acquire the Silent Assassin rating, even when the alert starts as you will be far and away by then.
Will Need: Combat knife or fiber wire, equipment pickup, anaesthetic(if you want to take the easier route).