My roommate finally got to the end of it a week ago. Fought the last boss. Had his party full-heal the final three times before he killed it. First time, Night Queen charmed Akihiko, and he healed her when she was nearly dead. Same thing happened next with Ken, aroudn the same time. Then Minato got charmed with Melchizedek on...and full-healed the final again. He thankfully stockpiled Somas, so just said "fuck it" and Armageddon'd her ass.
But yeah, go Narcissus, unless you really want to put amps/boosts or elemental resistance on a character. Abaddon is a better idea overall, though - don't know if your Devil is maxed or not, but make him, get him to like, one battle before his final level where he'd learn his last skill (assuming he has a heart), register, level, delete, repeat until you have full status-immunity for the group. Fun stuff.