The waypoint at the start of this mission is on the roof of a building. Locate the door you’re supposed to open and use Eagle Vision to spot the many guards around it.
How you proceed is up to you, but one badass method is to grapple over to just above the door and assassinate the guard directly in front of it while the others are looking away, then leg it through the door. If you have trouble with this and want to remain undetected, pick off a few more enemies first.
Inside you’ll find a friendly NPC. Trot around after him until he tasks you with sabotaging a bunch of valves on the floors above.
Optional challenge: This bit isn’t that tough but you need to be careful if you want the optional objective of not killing any guards except with gas; be very careful only to knock them out as you sneak around, and if a fight starts try to get away and hide rather than risk a fatal blow. If you must fight, make sure you use your fists.
The first floor is easy enough, but once you reach level two and three be aware that you can’t retreat to lower floors if trouble starts. When you’ve hit all the valves, pull the lever and head up to meet your pal.
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