Board Thread:Questions and Answers/@comment-23371555-20150813053115/@comment-26955746-20150813114456

Yeah, this is an issue that a LOT of stealth players had first time around. It is possible but it depends on what version of the game you're playing.

With the Director's Cut they added in a hack-based way to take Barrett out. There's 2 ladders in the room which can be used to access an area above. There's a computer up there that allows you to open a couple of doors to a previously inaccessible area. Go back down one of the ladders and go through one of the newly open doors. There's an office in here with another computer. Hacking this allows you to turn on 2 inactive turrets that you can use against Barrett. You'll either need to lure him into the location of the turrets or if you have the aug to lift heavy objects, just carry them to where he is.

If you don't have the DC you have to make do with the items around the room. There are several explosive barrels that you can throw at him. These barrels coupled with a few shots from any weapon you can find in the environment (its been a while since I played non-DC so I can't remember what weapon(s) there are) should be enough to take him down. Throwing the barrels can be awkward though since if he shoots it while you're holding it, obviously it will do a ton of damage to you.

There are also some other tactics here which might be helpful. Good luck!