Asassins work, too, but operatives have the added benefit of Toxin Scan (removes basically everything that sniper can throw) and Evasion, which dodges the Ambush at 100% chance.
Assassins have force shroud, game over. This thread is definitely in the realm of TL;DR but if you're either stealth class and you're having a lot of trouble with snipers then you should learn to play your class.