Belome the first...
"Is that a fire hose or his tongue?"
~ Mallow

I hope you had the guts to defeat the Pandorite. Of course, unless you're actually looking for it, the only way to run into the Pandorite is to be totally lost; I found this no problem, as I was born with the sense of direction of bread.  And believe me, bread has a worse sense of direction than you might imagine.  ANYWAY, the Trueform pin you'll win for pounding the Pandorite (I love alliteration!) will be quite an asset against Belome!  Equip Mario with the Trueform Pin if you've got it, and Mallow with the Antidote Pin. (If you don't have a Trueform Pin, too bad!  Nyaaaa ha ha ha!) If Belome eats Mallow, don't fret. He'll be back soon! And, if Mallow goes Scarecrow, his Thunderbolt will do the job quite nicely. Fire Orbs and Jumps can help, too....if they want....right.  Yeah.  Heh heh heh.  And never underestimate the power of regular attacks!