Wrathful Staff

Any of the mounted healers with an atk boon use it well; outside of that, Genny is pretty much the best straight-up damager even without an atk boon. You can get some healers with high enough speed to double, but their damage is pretty pitiful.

Elise or Lucius can do it pretty well, but I'd probably just keep it on Genny, she does high damage regardless of her boon.