Too me it always sound weird that Omega weapon and seymour and all (also from others, not just you) that they're 2 hard cuz I already trained everyone once I got there, so those were easy but the Arena monster not.
Okay you can Capture 10 of every monster, and then fight all arena monsters then you can fight Nemesis (much stronger than omega weapon) and when you beat all dark Aeons then Penecea is availlable to fight (I heard it was the hardest one in FF X) and to beat them all you better get all Celestial Weapons wich allow you to do more than 9999 damage ie. my Tidus does 99999
