Finally beat Redbeard on normal difficulty and it took 3 tries before I figured way to beat him. I can't imagine trying this on Torment. For people playing on normal this is how I beat him.
1. At start of battle, have Janell but the gems that give these blessing: Haste, War Chant. Then next round have her summon drake, use speed scrool, and cast Thorns.
2. Everyone should have Haste, Battle Frenzy, and Thorns now. Everyone should attack Redbeard with their most powerful attack. On normal difficulty at least, he is most vulnerable to magic/elemental damange. He has high physical resistence.
3. When Redbeard gets down to 1/2 health he will start healing and Sould Jars will start appearing. At this point have your pets go and click the buttons on the wall to open up the secret doors.
4. Position Shima and a bow BM in the right room and Natalie in the left room to kill the soul jars.
5. Position Janell and your remaining PC outside the entrance to the left room.
6. Start killing the soul jars and once the right room is clear move Shima to the left room to help Natalie kill the rest of the soul jars.
7. If you keep killing the soul jars fast enough in the left room, you should be able to pound on Redbeard with characters in the left room once all the soul jars are gone.
Once I got all the soul jars where gone, I was able to bring down Redbeard pretty fast with magic attacks. Even a dex based Janell was able to do a lot of damage with her Spirit claw magic attack.