Mack the Knife...
Well, first off, I say fight all the Shysters you can find in the kingdom and palace until you get a level up. Saving the innocent bystanders pays! Sometimes they'll give you something in return for restoring peace to their quaint little lives. Once you've gotten a level up and been down to the unguarded vault, take on MACK THE KNIFE! He's really not much of a problem. Use Mallow's Thunderbolt to neutralize the Bodyguards. Mario's Jump and Fire Orb prove effective against Mack. If FP is low and there are no Bodyguards, you can put Mallow in defense. Hitting Mack with the little guy's regular attacks does no real damage, and he'll be safer from Mack in Defense. Don't be afraid to just keep on whaling on him with Mario's attacks until he gives up!  That is, you can be afraid and not keep on whaling on him if you WANT to, but in that case your attention span is probably longer than mine. ^^()