It really don't matter, see? They don't got no force-fields, no area-detects, none of that shit. They'll know he's there when the most powerful of them drops, dead, because of a bullet. And by the time everyone gets defenses up to protect from attacks at all angles- for he will remain unknown, even after firing his shot- another two or three will have dropped.
If a knife can draw blood, a bullet can still pierce. And a real sniper is unmatched in placing a bullet.