The thing about FFX monsters, is that once you know what you are doing, you will easily pound them into the ground. Even Nemesis isn't hard after proper training, just annoying, and persistent.

One of the few exceptions is the Cactaur King (who counters nearly ALL moves with 99999 needles). For me, the hardest enemy is the one that can counter. But Bad Breath is bad too. Berserk + Confusion + Poison == dead. Sleep + Poison == dead. Ribbon == non-existant in NA. Auto_remedy don't work while Confused or Berserk, or asleep. Thank God it doesn't cast Doom or Death, just everything else.

Int has got it WAY too easy, I don't care that it has Dark Aeons, because Ribbon + 1/2 of difficulty for getting Zanmato == invincible.