Why are you leveling up still? You can beat the game, just fine.

If you must unecessarily max out your characters, than Grand Dragons or Yans are the best bet. Yans are very dangerous though, and if you get unlucky, you get Virus'd and don't gain any Exp. But use Zidane's Masamue to inflict the 'Death Sentence' status on them, for an easier kill. Grand Dragons can be offed by 'Level 5 Death' as well as the Masamune.