Cutlass

Every hit stacks a multiplier that tops out around half again your normal output, and it drains away quickly once you stop landing swings. Against a crowd you sit pinned at the ceiling the entire time. Against a single boss with long gaps between openings, you spend most of the fight partway up the ladder and never see the number the tooltip implies. There is a small mercy built in: the counter keeps climbing past the visible cap, so a brief pause eats the hidden surplus first before your damage actually drops.

It is shared with the Falcon Blade and the Exotic Scimitar, so you can swap between those three without losing progress. Swap to a bow, a summon, or anything else to finish a target and the whole ramp is gone. Plan to commit for the length of a fight.

Pirate Invasion. It is rare off the ordinary rank and file but much likelier from Pirate Captains and the Flying Dutchman, so kill those on sight rather than letting them wander. Clearing a whole invasion without touching a single captain still lands one a little under half the time.