Eraser bypasses Ribbon as well as Stoneproof, so that'd be no help either.

There are two main strategies. The Wakka-only one is described above. The other involves levelling Tidus, Rikku and Wakka, having Auto-Haste, Auto-Phoenix and Quick Hit, then just attacking as quickly as possible using Quick Hit, Attack Reels and Blitz Ace. If you're fast and strong enough you should be able to kill Shinryu in one round.