Defeat all the strong enemies
The trickiest part of this is the beginning, here is the order I recommend taking them down in the beginning, after this just take down any strong enemy that appears on the map.
Doflamingo
Sentamarou
Coby
Garp
Anyways here is the easy flowchart for this mission.
Put the difficulty on easy so your allies don't die and pick Boa first. With Boa do Treasure Event 1 and 4. After that quit, go back to hard difficulty and the character you play best with BESIDES Whitebeard or Luffy (they can't trigger all events) That's the quickest least stressful way to tackle this mission.