If you have more than a few Hi-Potions, sell all of your Potions so that your Hi-Potions are used when Auto-Potion kicks in. Use Steiner and equip these abilities: Auto-Potion ability, and Bird Killer ability. Make sure Steiner is at least level 8 to make it an easier battle. Use a tent on the monster, and it will take on every bad status effect that it doesnt guard against.

Catoblepas
Catoblepas remains in Treno until the end of Disc 3 or until you defeat the monster. Equip the Jelly ability and Beast Killer ability, and use the Auto-Float ability. Use a Tent on him until the beast bites him (You have a one in three chance of this happening) and he will receive every bad status effect that he can't guard against.

Amdusias
Amdusias casts Lv4 Holy, so avoid levels that are multiples of four. Equip the Antibody ability, and defend when it's in the air because it will only physically attack at this point. Equip Auto-Reflect if possible, because it only uses magic on the ground. Also, equip Auto-Regen or Auto-Potion. If you have Reflect, Amdusias will kill itself and you can just defend. At this point, use a Tent on the beast until the snake bites him (once again a one in three chance) and he will receive every bad status effect that he can't guard against.

Behemoth
This monster is extremely difficult to defeat. It's best to use an attack that kills the Behemoth instantly. You can use the Blue Magic Roulette or Zidane's Soul Blade attack when he has the Sargatanas weapon equipped. This will instantly petrify him otherwise you need to just continue healing and pounding away at this creature.