I cheesed the Dr. Wily fight with Ness — I probably took enough damage to kill me two or three times, but endured the entire thing by putting up PSI Magnet every now and then and watching the Mega Men shoot pellets into it, repeatedly, like complete dumbasses (instead of doing what an actual human would do and running up to grab, Top Spin, Mega Upper, slide, Spark Shot, or Flame Blast). I did get punished with Metal Blade or Leaf Shield occasionally, and I think the Mega Men got smarter as the match went on (the last one picked up a beam sword instead of relying on pellets), but for the most part, I was able to regularly heal from near-dead to full health throughout the battle. It was nuts. I also used Ness to beat the Boss, though it took a few tries, and I ultimately won by knocking her off the stage and footstooling her.
Also, I can confirm that using a Staff on Pauline instead of playing the "chase Peach around the map" game works wonders.