The three special stage bosses only take damage from the normal buster + their weakness --- red joker:3867-2193-4043(japaneezy) black ace:4640-6968-9201(english) platinum 2493-0945-8023(for trades only) soul silver 2579-3916-1665
I actually had less trouble with Ballade than I did with Punk. I could never predict whether Punk would aim his Screw Crusher high or low. So sometimes when I'm close to him I think he'll aim low, so I jump, only to jump right into a high shot.
I think it's actually harder to beat Punk WITH the Mirror Buster than is to just use the standard arm cannon.
My guess is that this is a very, very purposeful design choice.
By positioning Punk and his stage as one of the least difficult in the game, and awarding you with a weapon that basically replaces the buster for anything above or below you, the developers are giving one final leg up to everyone who's still having trouble with the game.
Getting the Screw Crusher makes getting the other special weapons a cinch, and after that playing through the game as Mega Man is infinitely easier. If you still can't progress through the game after this, then games that test reflexes and hand-eye coordination just aren't your forte.
--- "All opinions are not equal. Some are a very great deal more robust, sophisticated and well supported in logic and argument than others."
Punk's stage did have those tricky jump between see-saw platforms (if you don't use Rush), and Punk himself does quite a lot of damage with his attacks. So, no, I don't think the designers intentionally made Punk easy. --- Knowledge is power, and ignorance is bliss. Unfortunately, most people won't settle for one; they usually end up with neither.