I've only ever had trouble with Yunalesca on my first play through. Not attempted any of the Dark Aeons yet but I can annhilate Valefor and Ifrit but I just need to get HP at the moment. I'm also waiting until I get an armour with Ribbon. I've got 255 on strength, defence, agility, magic and magic defence. I only need HP and a bit of a booster on MP. I've beaten Nemesis after completeing the monster capture quest and found him quite easy after getting a routine going.

The easiest way to beat the bosses is to just use Wakka's attack reels. 99,999 x12=1199988 the use Entrust with the other 2 characters and you can do 3599964 which can take out most bosses.