Below you'll find a walkthrough for A Bitter End, a mission concluding the ninth sequence of Assassin's Creed III Remastered.
Optional objective: Limit environmental damage - 20%.
After meeting with your father, you will take control of the ship and will have to sail between Caribbean isles. Start off with half sails so that you can maneuver and avoid damaging the ship.
Optional objective: Stay within distance of the opponent - 500m.
Optional objective: Complete all optional objectives during a single playthrough.
As you reach the abandoned ship, a bit further you will see the refugee. Start chasing him.
As you're sailing through the rocks, it's most important to manage your sails. Because of the strong wind, each time you go full sails, the ship will turn very sharply. You can use it to quickly avoid obstacles. If the wind is bad, go half sails and maneuver the ship. You need to get the most of the wind so that the enemy doesn't escape.
After reaching the designated spot, you will receive a task of destroying the red enemies. Attack them with the cannons and swivel gun while looking out not to hit the big ship.
After destroying the last enemy, fold down the equipment button and choose the Chain Shot ammunition.
Using them, you will have to break the mast of the last ship. Shoot it by holding down and then letting go of the run button. Eventually one of the shots should reach it.
After you manage to do it, slowly approach the ship and jump aboard.
Optional objective: Achieve a minimum kill streak - 3.
Your main task will be now to kill the three officers marked with red. Additionally to complete the optional objective, you will have to perform a streak of 3 kills.
To do that, run to the commander in front of you right after getting aboard the ship.
He will lead you to the end of the ship, where you will be attacked by a group of enemies. It's the best place for a kill streak.
Another good spot can be found on the other side of the ship. It's also where the other officers are waiting for you.