After having cleared the maze to reach this fight... it may feel a bit underwhelming compared to the difficulty spike you just went through (unless you used the Out Of Bounds glitch to fly up instead).
-Note 0: By now you better have all upgrades purchased and all items maxed out (barring healing items, as this is a No Damage achievement)
-Note 1: Bubble Shield, Super Monster Milk, and Super Pike Ball are a MUST at all times during this fight. Especially do not go for more than a moment without a Bubble Shield, it is your very best friend.
-Note 2: The Pirate Master has 1000 HP.
The Pirate Master has a rather direct pattern of attack:
-1: Teleport Phase - He teleports several times, between left, right, and center positions. First is 4 times, then 2 more times that release attacks (blocked by Bubble Shield), then once more (always in the center) as he summons the platforms for stage 2. Just don't accidentally run into him and take damage!
-2: Platform Phase - He summons two platforms then teleports to stand on them, swapping sides after making a wide ranged attack. The Bubble Shield will keep you safe, but be very careful if you try to attack him, as the platforms are swaying and can bump you into him! (He covers the WHOLE platform that he is on, so you can NOT stand "next" to him for safe attacks. Just stay to a side and fire your Pistol. The best method for some extra damage during this part is to either do a single jump and let your Super Pike Ball hurt him or to double jump on a side he's not at and fire your Pistol a couple of times; the Super Pike Ball is definitely the better choice.) Whatever you do, DO NOT be standing by either side of the area when he's done with this phase as he will teleport to either side for the next phase!
-3: Laser Phase - He comes to the ground and then some targeting crosshairs appear, telegraphing the laser attack. Your Bubble Shield will protect you here as well, so tear into him!
-4: He returns to step 1 and just repeats in order until you defeat him. If you're careful, you can get him down in a single cycle.
Yay?! (Did you earn the Bad End or are you moving on to the Good End Battle?)
Special Note: Whether or not you take damage in THIS battle, as long as you END this fight with full HP you can still get the No-Damage Achievement in the True Pirate Master battle. Confirmed by myself.