I used Yuna, Cloud, Squall, Kain and Lightning. What didn't help was that my random assist was Shantotto, ended up only using her to block HP attacks.
With Yuna I tended to use Ifrit to wall-rush him and follow up with Mega Flare (which he seems to stupidly not avoid too well). This actually went well until he eventually got me at about 60k HP.
With Cloud I wall rushed him with Sonic Break, Slashing Blow and Aeriel Fang until he got Break, let him recover (gives a bit of a BRV shield should anything go wrong) and do the same again and follow up with Omnislash V5 or Finishing Touch. Got him to around 25k doing that.
With Squall I could hardly land anything at all, gave it a few tries and ende up chipping off ~5k.
With Kain I used Celestial Shooter & Crashing Dive the same tactic as Cloud. Break him and eventually hit him with a HP atk. I found Sky Rave easiest to go with. Got him to about 3k using Kain.
Lightning simply finished it off.
I found EXP to HP and Riposte extremely useful as far as abilities go.