To my knowledge all bosses and Lords have some level of damage reduction. This could be increased and/or perhaps introduce a damage capping mechanic for bosses and Lords that simply does not allow the player to deal more than X damage within 0.something seconds. I think this would work best if the damage was % based rather than a flat number.
Also, I do not know the time interval between GK’s ult’s first and second hits while dual daggers count as 2 hits and the same goes for BH’s Double Shotted, hence limiting damage per hit rather than a short time period would leave us with outliers.