Theoretically a champion that just set your Ability Accuracy Attribute to zero at the start of the fight would probably be packing the strongest single ability in the game, but that currently doesn't exist as far as I'm aware.
It somewhat exists. The champion is called Mephisto. No buffs activated at the beginning of the fight, also counts for node buffs. Node buffs don't even pop-up unless re-triggered. If the node buff is a permanent one, it is simply gone. It helps a lot in lots of quests.
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but i don't know if its a bug or not but Dr.voodoo bypass this by activating his regen at the start of the fight