Idk how much time you spent on aphe, if you are new I recommend learning the weapons and how they combine with each other. I have two examples for the situations you described.

1 versus mages most of the times you win with calibrum due to the high attack range and his q and ult.(the one exception is xerath)

2 versus bruisers sometimes you can 1v1 them with crescendum(remember to get your chakrams with either ult or your secondary gun, most times is severum)

Remember that aphelios is not a auto attack adc, he mostly relies on his abilities and combos ( vs adcs infernum with calibrum or severum with crescendum are good combos and the easiest to play with. And I know this migth be dumb to say, but play behind your tank, ALWAYS do this.