Ffx overdrive sin help?

Sounds like you should be strong enough. The fight is all about just unloading as much offesne as you can in as short of a time as possible. Get everyone's overdrives charged up (including Bahamut's), and start the fight with Yuna paired with Tidus and Wakka. Yuna should Hastega her first turn, while the other two use their best overdrives. Swap out Wakka and Tidus for Auron and Khimari, let them do overdirves as well, while Yuna Hastegas again to let them tag in Lulu and Rikku faster. Lulu and Rikku then also do overdrives (Lulu should do Demi if it works on him, and Flare otherwise; Rikku should aim for damage or stat boosting with her Mix), and Yuna again Hastega-ing. Then just unload with your best attacks, and when Sin is getting close to doing his instant death, Grand Summon Bahamut and have him Overdrive as well.