1. You must play as Hancock and defeat Sentamarou before fighting Garp.
2. Doflamingo must be defeated while Crocodile is nearby. If he isn't, then lead Doflamingo towards him.
3. There will be Marine groups with the “?” icon. Defeat all of them before they can takeover Whitebeard's territory.
4. Quickly run to the scaffolding and clear it. You have to be very fast. You'll know you got it in time because the “?” icon will appear over Garp.
5. Once Ace is freed, you will have to deal with more Marine Captains that have the “?” icon.
6. Playing as anyone other than Luffy or Whitebeard, defeat Sentamarou when he retreats to his Pacifistas. He must be defeated before they are.