There are lines around Pentel that hold up a chandelier over his head. Cut those ropes and the chandelier will fall on top of him. This is much faster than throwing objects at him.

Use Jack's Grog Bombs and throw them on the platform..

When he is near the central mast, he must be attacked at the same time so that he does his combo and damages the mast.

Attack by pressing Square(2). When he attacks, block with R1. Finally, when he hides in the smoke, pick up a barrel on one side and on your way to the other look for bullets or bombs coming out of the smoke. This is where he will be. Then throw the powder kegs at that location and pick up the one on the side you are going to. Repeat this until he is defeated.

First, upgrade all your moves that you have. Then kill as many enemies as possible. You will easily kill them. Repeat the level, then kill them again.