One hit kill on Legendary!Asclepion said:"Sniper Alley" in the second level has at least eight, enough so that on harder difficulties there's virtually no chance the marines following you will survive, even if you can get past them.
I actually got all the way to the Elite rebellion on Legendary, but there was a bit where you have to snipe some Jackals across a wide clearing. Sniper the first one, a second spawns immediately behind it, facing you, ready to fire. I tested it a few times and the damn thing can head shot you faster than you can fire a second shot or move to cover, I'm not exactly sure how you are supposed to ever get past that bit of the game to be honest, luck?