So I think overall my aim is pretty decent. What I struggle with is long range shots and hitting strafing targets. Whenever I hit a long range shot or someone strafe shooting, it seems to be mostly luck. How would I go about training this?...

Riot needs to incentivise that kind of gameplay within the combat score. It makes no sense that they changed Skye's flashes to incentivise teamplay, but if you flash an enemy and give your teammate a kill you get 25 points while they get 30...