I noticed a lot are having problems with the boss. Here's some simple steps to help you succeed with ease:
1. Lower your sailing speed to avoid walls. Do NOT try to avoid shots (requires high armor and tortoise shield).
2. Kill the corners to unlock the center piece. This feat is can be done (requires Galley, ramming or revenge help as well). Just lower speed to 0 at this point until the it dies, and go to the next (ram your way).
3. Kill center using fireballs- it takes most damage from these. Just circle around it, do NOT enter the inner circle, as it damages you rapidly. You can take hits freely as long as you keep hitting him.
Thumbs up to keep alive.