Play defensiv at the begining and try to farm as much as posibel.

Hi sorry this is my first so be nice...

I go magic resistans glyphs counter the ap champ i meet. So if he gets a spell on you your stay a bit more healthy.
I go armor seals for late game and if they have a jungler.
Magic pen marks so i can farm better and deal better damage late game.
Ap quintess for the damage.

Masteries i go 9 14 7 focusing on ap defens and mana regain.

I focus most on ap to deal tons of damage in team fight but also a lot of health and defens so i can survive easyier and so i don't get to squishy

I think that every champ need flash so that avius. And the clarity is becuse she use here mana a lot and to keep fighting as an ap champ you need mana so clarity is what i always pick for ap champs.

Just try to put your W in the middle of big minion waves and try to have ap then it will work.