I think I breezed through the whole pit by abusing Koops and his attack that goes through enemy defense. For the floating enemies that are annoying, keep using Flurrie's gale force until they get blown away, instantly defeating them. When you get to the bottom, you fight a big ______. He's supposed to be the toughest boss in the game, but it was really really easy for me. Here's how I did it.
All you need is the piercing blow badge for your hammer and goombella's rally wink. The piercing blow will do damage every time because it ignores defense, and more importantly, you can backflip like 4 times if you time the appeals right and get a whole star point every turn. So just do a piercing blow with Mario, give Mario another turn using Goombella's rally wink(don't forget to appeal with her too), and use a sweet treat to regain any FP or HP you lost. You can do this forever without running out of star points because of all the appeals. It helps to speed up the fight if you equip other badges that power up your hammer too.