At any time when both Jack and Will or Elizabeth are injured, find a health chest. Open it with one person. Instead of moving that same character, switch and walk a bit off so that the other character moves with you. Have the person you are controlling move back to the chest and get the power-up. Switch back and both characters will be fully healed.
First, upgrade all available moves to make killing enemies easier. Then, kill as many enemies as possible. Repeat the level, then kill them again.
Attack Blacksmoke James, and when he attacks, block. 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.
When he is near the central mast, attack him so he does his combo and damages the mast.
Use Jack's Grog Bombs and throw them on the platform.
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.