Protip: King Bubble Slime can appears as a regular enemy on the Hard difficulty. He's.. just as hard as the Hard boss, just without the boss-strength groupies.
Also, the Hard map, if you can handle it, has an excellent Stamina to Exp ratio, but decent gold. I don't mind grinding this one over and over, worrying that I am wasiting precious exp-gaining time.
Here's my Bubble-killing Party:
The Marquises Freezing Blizard, the Elysium Birds Thin Air, and my Pal (Marquis de Leon or Balzack) will Freezing Blizzard and/or Crackle, depending.
Mucho Macho isn't that useful, but he oomphs himself and picks off the goons with regular attacks and/or Sword Dance, so he works reasonably well.