My characters were very low when I beat Dark Bahamut. I just use Yojimbo's Zanmato to kill all the Dark Aeons <img src="/images/graemlins/wink.gif" alt="" /> . Now I have Dark Yojimbo left <img src="/images/graemlins/laugh.gif" alt="" /> .
Just take Yojimbo to Zanarkand and call him on every battle you come across. Pay him around 1000 gil or more and his affection for you should grow. Also, don't let him die or don't dismiss him or pay him 0 gil - he won't like this <img src="/images/graemlins/mad.gif" alt="" /> . Then take him to the
Evil Dark Bahamut and hopefully Yojimbo will chop him in half

. Try and pay him as much as you think is neccessary.