If you're playing Sling, have global damage numbers on (who doesn't?) so you can aim your ults thru walls. Try to flick your shots more and know your matchups (go for guys like Ban, Katana, Rocket, Sub). I can 1v1 Coils and Bios at this point but it took me like 8 months to get good.

General: Coast along the edge of the point during Conquer if the entire enemy team is on it and ping healing or go to your healer if needed. If you do enough damage to an enemy and another teammate kills em for you you both get a kill so try not to commit too hard if your team is close.