% armor penetration doesn't stack in a 50%+50%=100% way, it stacks in a 50%+50%=75% way. The first 50% will penetrate 50% of a targets armor. The second 50% will penetrate the remaining 50% by 50%.
A player would need to take advantage of something like Yasou's 50% bonus armor pen post-ultimate to even come close to 70% penetration, likely with smacking someone 6 times with Cleaver. And if you can survive being hit 6 times and taking his ultimate, you're tanky enough to be an effective tank come late-game.
Also: Dominik's is 45% vs. Bonus armor, not total armor, which makes it completely useless vs. the innate ~75 everyone gets from bases. This has a pretty profound effect on the armor penetration it actually offers, and prevents it from being added to Black Cleaver's penetrations into 1 big chunk.
It's also worth pointing out Cleaver needs to apply 6 sources of physical damage to get 30% armor penetration against a target, only actually giving the full 30% penetration on the 7th physical damage application.
Completely negates the defensive stat's of armor items. What the fuck am I suppose to build?
It does not completely negate defensive stats, due to the flaws listed above.
Such a build also looses out on a lot of DPS potential that attack speed and critical strike chance would otherwise offer, resulting in fairly low overall damage that could be countered fairly effectively with health alone.
Beyond that: you're describing a very late-game build of a carry.
Carry is used to describe champions that are so weak early they need to be carried by their team, but scale so well with items late they can return the favor.
The obvious solution to beating them is not letting a game drag into overtime, which is ~33 minutes looking at game-length averages.