Bonus Damage Protection factors into the Battle equation and is used to mitigate the effects of Bonus Damage.
A very select few things generate protection against Bonus Damage in Age of Empires Online.
Milestones[]
The level 40 Norse Milestone Adaptiveness reduces incoming bonus damage against Infantry Units, Ranged Units, and Cavalry Units by 25%.
Gear[]
A list of gear that provides Bonus Damage Protection may be found here: celeste-search.netlify.com