Why would he mean Sin when he clearly said Braska's Final Aeon: Jecht? Sin and Jecht are two totally different battles. He's talking about the final boss.

Anyways, if you can't beat him now, and you're using strategies... Your best bet is to either work on your Sphere Grid, or get the hidden Aeons and previously get all their Overdrives up. Obtaining the Celestial weapons plus their sigils and crests would help too. In fact you should spend some more time doing sidequests and stuff. But if you don't feel like doing that, here are some key points...

- Cast Slowga. (preferably before and after destroying one of the Pagodas)
- Save the Talk command for the second round.
- Cast Hastaga.
- Wear armour that protects mostly against Petrify or use a soft right away.
- Have Protect on everyone.
- You can use Armour Break, Power Break, and Seed Cannon if you like, but it wont help for long.
- Don't be afraid to use Megalixers.
- When his overdrive starts to get full nearing the end of the second round, use your Talks. Or get an Aeon in there.
- Quick Hit helps if you have it.