To unlock this achievement, you need to kill 50 players while they are looking down the sight of their scope. This would be better boosted, as you would struggle to notice someone using their scope if they where at a sniping distance. You can look for players hiding away who look like they are sniping and kill them. This needs to be kills on people using the scope of a sniper rifle and not using the scopes of assault rifles or SMG's. You can track this statistic by looking at your Agent Record and looking for the combat ability, Counter Sniper. The achievement will unlock after your 50th kill.