He only heals himself once, so don't be disheartened. However, after he heals himself, his attacks become stronger and more frequent.
You can't debuff him, so buff up your party at the start or use some shrouds. Haste and Protect are extreamly important, so at least make sure everyone has that. Switch into Relentless Assault, and go for it to stagger him. If your health does down, switch to Diversity to heal, and continue to pelt him with everything you have. You should be able to stagger him before he changes form, so you should be able to rack up some serious damage if you switch to Aggression (or stay in Diversity if your health is still shaky).
He'll heal himself when his health goes below halfway. I suggest switching to Riot Shield (with Light/Hope/Fang that's MED/SYN/SEN, right?) or Combat Clinic at this point to get buffed up again, and any last minute healing. Then switch between Relentless Assault and Diversity until he is staggered again, then let loose with Aggression.
When things look bad, don't forget about Renew and your Eidolon! Renew has saved my skin countless times, and Eidolon's are good for 'renewing' and getting some damage done at the same time.
Good luck, and I hope that helps.
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