Superspeed overrides the slow inflicted by holding a treasure chest.
/gg to reset mobs on the ship and use the Mistlock Singularity to stack boons. The two mechanics from before, the knockoff and AoE are present in this fight as well as a third one. It is an AoE placed on a player with a timer. If the circle fills, huge damage is dealt to the player, but you can split up the damage between players, to counter this mechanic simply stack in the circle to split up the damage dealt to each player. The boss will be teleporting around as well as spawning mobs every 10% health of the boss. Kill her to finish the fractal.