Super Mario Boards

Repetition and practice is your friend. Learn the boss cues, learn when to attack and stop attacking (and start dodging), shield when you can (but in some cases dodging is better or even taking damage does less damage than shielding). I used the Hammer ability too. I don't use the firey flip too much, it's too unsafe. Instead, I just used the very basic attacks like pummeling and the midair Hammer attack pretty often as well as the Hammer dash attack.

If you want to be cheap you could just use Stone. It's very long and tedious that way but it's a guaranteed win if you're careful. Hammer is also really good, using the dash attack with it makes you invincible until you stop spinning, so this is also useful.

Pretty much what the other two say. Just keep playing over and over, learn the boss attack patterns, and you'll eventually do it one day.

Hammer sounds pretty good. I'll try it. Who knows, it might be even more effective than Plasma's been.

Hammer is actually very effective if you want to get things done quick. You know what they say, the best defense can be a good offense. Hammer can kill some bosses quicker than you expect, and if they die sooner, it's less opportunities to get hit. Stone is also a good ability, but it's pointed out that it requires a lot of patience. Don't get cocky with it either.

Hammer's done me pretty well so far; on several occasions I've made it all the way to Galacta Knight. Hammer is a bit tricky to use against flying bosses like Kracko and Kracko Jr, but I've mostly gotten the hang of it. Before long I'll have beaten Galacta Knight, leaving only Marx Soul to deal with.

But the flipping attack midair still does a fair bit of damage. You tend to risk teleporting characters hurting you, but again, it's pattern recognition that'll be your friend, coupled with Hammer just making short work of a lot of bosses. You might be tempted to use the fire flip, but iirc, every attempt I made usually ended up me in a very vulnerable spot or getting hit.