I think the Engie class fits sniping well, combine it with a turret to make the enemy dodge predictably, or with a mine to weaken them first.
Although it's not a one shot kill, remember that it's not a one player battle, use the time between shots to work out which enemies are going to be aimed for first, or which are already taking fire, then soften them up or finish them off respectively.
Not that I'm any good at sniping.
I might make a decent spotter though.
Sniper Medic with Spotter Engineer?
Medic uses Barrett (more firepower) while Engie uses Drognav (better reload time). Basically, Engie will potshot at enemies, but his main purpose is to mark target for close-in friendlies (because you can do that just by centering crosshairs on them). Medic will pick his shots more carefully, and make every one count.
Engineer can also set turrets strategically to prevent enemies from getting into any safe zones that would prevent the sniper team from targeting them, and mine the way into their position (at best, this will slow enemies down, at worst, you'll know when they're coming), while the Medic can patch them both up if they start taking fire - at which point it's time to move on.