If you're already a good sniper in PVE, i'd say learning the maps is the most important thing. Where to rest your weapon. Where enemies will come from to gank you from close quarters. Knowing when you round a corner, exactly where you need to be aiming. When someone is already aiming to a spot that you are going to, you need to be on them in a blink b/c that is the reaction time you are dealing with. We all have it. Just need tons of practice.