Yeah, maybe you just need to progress further through the sphere grid. I think that an easy way to beat most bosses is to use your overdrives. Your characters and the aeons. Magus Sisters are great to have also. They can do a lot of damage. You should pretty much be set if you do everyone's overdrive. If you still have problems, get the celestial weapons and maybe level up a little. Also maybe you should upgrade your armor. With some good armor you can stay alive for awhile. Good luck with beating Sin.