"A Bitter End" is a mission in Sequence 9. It helps to have the Aquila decently upgraded or fully upgraded with Cannons and Iron Hull Reinforcements at least.
Limit Environmental Damage: 20%
Stay within 500 meters of the schooner
Achieve a Kill streak of 3
Find Church and his ship 'The Welcome'
Chase Church's schooner
Sink the Man-of-War's Escort
Destroy the Man-of-War masts
Board the Man-of-War
Kill the officers aboard the ship
The Foam and the Flames