Unless your dedicating a run page to shaco/plank/ashe do not stack crit dmg, i made t he mistake, i found a few forums and asked a few question and decided to try armor pen, the difference is unreal.
i prefer magic pen with kat than armor pen
You know when they change how armor penetration works it might be more benifical to get Crit Damage.
also viable for pantheon and tyrn imo
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