ehm.... well i dunno what the other guys think but you should atleast have 3 or 4 times as high stats as you have now so i suggest you go to omega ruines and train there for a while, make yuna go down aurons path so her strength goes up ( so the aeons get stronger too ) this might me a goo idea for tidus aswell since hes not THAT strong, if you want auron stronger then he is now and he's finished his own path make him go down wakka's path since wakka has alot of strength +4 nodes on his path
just keep training until you think theyre right and just try fighting a dark aeon, youll see if your string enough for yourself.
( if your using the expert sphere grid then just look for where the most strength nodes are and send them there )
geeeez