if you know where the support is harras him with your Mistic Shot and as well the adc when he dont stays behind his minions. DONT use your Mistic Shot to last-hit , its just mana waste .

Trueshot barrage is just awesome if you know how to use it :

get the kill on another lane : shoot your ulti torward an enemy tower where the fight is

get the kill in a teamfight : use your arcane shift backwards to get range and not get focused in the first moment then shoot your ulti and join team fight again.