How does it work on Gnar?

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the active movement speed is great for kiting in mini form, and the extra attack speed lets you get the 3 hit W passive faster. Personally I wouldn't build it because it really is beneficial on mini Gnar and not mega Gnar. The armor pen is also really nice. I build Black Cleaver followed my straight tank.

That's what I do too xD Cleaver Sunfire Frozen Mallet