To unlock the Pirate Master's true form you must defeat the Pirate Master's initial form after destroying all 20 Tinkerbats and collecting their dark magic.
It is recommended to enter the fight with as many bubble shields as you can carry, as well as some super pike balls and monster milk. Load up on these, especially if you're attempting the cheevo to not get hit.
The fight itself can be broken up into three phases, with the Pirate Master employing a set of attack within each phase.
Phase One: The Pirate Master will begin the phase by lowering his head to the bottom right of the screen (whack him a few times) while launching some cannon balls. Run to the right of the screen where Risky Boots is and as soon as the first cannon ball passes over head run all the way to the left, and do the same thing to avoid the cannon balls as the return your way.
Next the Pirate Master will summon scimitars above you, simply find the opening between them before they drop.
After that pistols will be summoned in the center of the screen and will begin firing at you. Activate a bubble shield to avoid all damage.
The last part of the phase will begin with the Pirate Master summoning platforms with shades on them. Simply jump on each platform, attacking the shade mid jump (super pike balls and your bubble shield make this section easier, as the shades will launch energy balls at you) and wail on the Pirate Master once you get to the top (Super Monster Milk makes this faster).
The attacks will repeat until you take him down.
Phase Two: This phase follows the same format as the previous stage with a few changes.
There will now be four cannonballs, though this doesn't change how you avoid them.
Scimitars will change position above you, so follow the gap.
The pistols will launch themselves at you after firing. Stay crouched in the corner with a super pike ball and bubble shield to avoid damage.
Phase Three: In this phase you will be able to shoot the cannon located in Scuttle Town with B. Keep the Pirate Master dazed with attacks and a super pike ball and shoot the cannon when it's aimed right at the Pirate Master's head. Be careful not to miss, as if you do so two many times he will shoot out a bunch of energy balls. These can be absorbed with a bubble shield though.