Max out Cannonball with 30 points after spending the rest of your points in other skills. Consider putting extra points into Bomb Throw. It allows you to throw three expanding rings of bombs around the pirate. The X Marks the Spot skill is also useful. You can set the trap on the pentagram from which the Boss spawns whenever that skill is off cooldown. The traps stack, and can instantly kill the Boss. Also fill the Explosive Bullets skill to increase your damage.