The Black Cleaver needs to be "Melee Only"
I still don't understand how an item that is inherently designed for Juggernauts still scales so well on Marksmen. Marksmen have plenty of itemization for Armor Penetration that Juggernauts wouldn't normally buy into. You see plenty of Assassins abusing this, but..
Another alternative would be a Warmongs-like effect. Where the use needs X Maximum Health before they can access the Armor Reducing. This would shunt Assassins and Marksmen away from purchasing it.