For this battle, I used Auron, Tidus and Yuna (with either her or Bahamut having a full Overdrive meter). I also customised their armour with Beserk Ward or even better, Beserkproof.
What worked for me is to place Auron in front of the boss, and send Yuna and Tidus behind it. That's when I started attacking. Auron has a great deal of HP, and doesn't really to be leveled up a lot to achieve this, so he can endure the boss' counterattacks for quite a period of time.
I found that the key was to keep Auron in front and Tidus and Yuna behind (this could involve a few turns of no attacking to make sure they're in the right position). Have Yuna constantly heal, and Tidus and Auron attack, using their Overdrives at every opportunity. You also have to watch out for the boss' Beserk Tail, because if it suceeds you can be in quite a bit of trouble. Have Yuna cast Esuna on whoever becomes afflicted with beserk.
I used Bahamut's overdrive when the boss got down to around 20 000 HP, then continued attacking until the boss was defeated. Also, remember to move off any space where a Glyph Mine has been placed.
Fighting the battle this way can take some time and a couple of tries, but it did work. Good luck!