XIn zhao is a really good ganker what that means is that you should switch between lanes and get as many kills as possible early game. But DONT forgot to farm! You will get underleveled and you'll do less dmg/have less hp.

Armor penetration is a need for almost any physical damage champion in my opinion so armor pen is top priority rune in my build.
I choose attack speed for seal becouse xin zhao works really well with attack speed, thats it.
Cooldown reduction is really good too for xin becouse he got alot of mid cooldowns (about 15sec cooldowns) and i choose per level ones becouse this build focuses on making xin zhao the late game star.

Well, what can i say. The masterys is the obvious choise i guess. =D