Red Dragon in Gurgu Volcano is quite dangerous as far as I remember. He hits for a lot of damage and his fire attack hurts even more. The Eye is a good enemy, but it's a bit hard to get to the bottom of the Ice Cave and he can hit you with instant death spells (although he'll hit you very rarely if you're equipped with ProRings). My favorite enemies for leveling up are Zombie Dragons in front of the throne in the Castle of Ordeals. They'll give you both EXP and Gil quickly, you can encounter with whenever you want to and they aren't too dangerous or hard to kill.