Use the Broughton 1000 Air-S shotgun; 5 shells and it one-shots up to about 40m anywhere on the body. And it'll 2-shots from basically anywhere you can still land a shot. Be advised, however, they don't stay knocked out very long on the two-shot KOs. It's a good idea to bring at least one leathal gun, however. I like a lethal sniper, non-lethal sniping seems to draw just as much attention. So you might as well bring something that can take out security devices.

You'll want to keep a very low profile against snipers, in general. Not only do the snipers see further, but they seem to be more sensitive to any kind of noise or suspicious activity. I don't recommend killing guards, unless you absolutely have to. In fact, I would try to make it as far as possible without damaging or stealing anything so you don't have to deal with the defender. Unless you need resources than do the security challenge and steal everything you can. I find there seems to be less defenders online in the challenge mode. Either way; you can fulton stuff knowing that the defender is either coming no matter what or not at all.

Someone mentioned crawl a lot, you can roll to cover ground quicker too without being any easier to detect. Use magazines and tranq rounds for diversion. I don't recommend going out of your way to do CQC, you not only have to get close buy you commit to being locked in an animation. That's not great in general and really scary against snipers. I'd save it for when you happen to run into someone on a catwalk or on the perimeters.

Here's a couple of vids: https://www.youtube.com/watch?v=-FqNJbzKaCM https://www.youtube.com/watch?v=xc78_Bi9oAM

The blocked bridge the second one refers to is when there's a bridge to another platform blocking that speed run up the steps in the first. You can practice sprinting up those stairs and get a good feel for the timing so you can dive at the end so you're prone when your invincibility wears off. Just be aware it does cause some suspicion with the guards, but it should let you clear the first deck in a matter of seconds when you get it down. I still don't recommend the crouch sneaking like they do, that seems to really get the attention of the snipers. I realize both of those are against lethal snipers.

The only advice I can give for non-lethal opposed to lethal is to take a little more time and KO guards with guns. You can still use stun grenades, but be careful of course.