Edited my above recommendation because I was corrected on a part of the physical mitigation formula between Attacker and Defender of different levels.
You physical mitigation from armor uses the Attacker's level (in this case the bosses) to calculate itself. And since the bosses are higher level then 40, you need more Armor to compensate.
Corrected the recommended armor value to 4730 - 60 per 1% reduced armor penetration
