There is one move that is an ABSOLUTE NECESSITY in this battle and that is Mario's Special move Clock Out. Start the battle by using this spell and make sure you press the button that appears near Grubba instead of the one near Mario and your partner! Seriously. Once Grubba is paralyzed beat him like nuts with Power Bounce and Body Slam or Ground Pound depending on whether you bring Flurrie or Yoshi who are both good choices for this fight. Goombella's Multibonk is also nice. Grubba has a whooping 60 HP which is a real lot. When he does regain his movement he'll be pretty tough I can assure you. He's slow but really strong. What he'll do is cast a spell to give him two turns every time you and Yoshi/Flurrie get finished and then increase his attack power by 2 and then do lots of damage by flipflopping all over you. However you got such a head start on him that this won't be a problem. If necessary just use Sweet Treat or even Earth Tremor.