Mission 5: Deep CoverThroughout this mission it's recommended to just walk as running appears to alert the enemies. Walking near enemies and becoming suspicious (Yellow Diamond) is fine as long as the enemies don't become hostile (Red Diamond).
Soon as you start the mission and have control of Kate the timer will start. Instantly holster your pistol by pressing
, then mantle over the two sandbag walls directly in front of you. Closely follow behind the LTV, giving a wide birth to both the left and right patrols that appear. At the first corner cut directly through the hanger in front of you and out the other side. Turn left and head towards the security checkpoint, the guard near the back will start walking forward, so bare left, go behind him and through the checkpoint. Walk into the courtyard, you'll see a guard walking up some stairs between two vehicles. Head towards those stairs, be aware that another guard does patrol between those vehicles as you approach. Squeeze past him, go up the stairs and kill the female guard in the room silently
. Within a few seconds the Major should follow you up the stairs, equip your pistol
and headshot him as he walks into the room. Hold
to loot his keycard. If done within 90 seconds the achievement should pop. See my video guide for more detail: