Orphan's biggest weakness is that he's very easily killed by poison. Once you get to him, restart so you can set up your team like this:

Vanille/Lightning/Hope
SAB/MED/MED
MED/MED/MED
SAB/MED/SYN
MED/MED/SYN
If you want, teach Lightning the sentinel role so she can reduce damage while Vanille spams poison and Hope heals you.
Also, having rings that will reduce the chance of death hitting you helps a lot.
Anyway, when the battle starts out, switch to the MED/MED/MED role to heal. After that, switch to the SAB/MED/MED so you can hit Orphan with poison while keeping your party's HP up. It'll take a bit, but once it hits, his HP drains quickly. After that, switch to the all med paradigm or MED/MED/SYN if you want to buff your characters. Repeat when he uses his ridiculously powerful attack.

It'll take a bit, but it worked for me both times I fought him.

DrAwesomeMD

 Same Party I had when I fought the Final boss..