Hell Wyrm to begin with has alot of HP but you can fight him several times you can leave the room where it is & come back & it won't of restored its HP. Its physical attack does about 2000 - 3000, it likes to cast Thungaga, Blizzaga, Firaga & Darkga well at least thats all I saw but I have a nasty feeling he may have a high damage spell like Shock or Scathe in its arsenal not sure. The main thing you need to worry about is its attack called Judgment. It causes 6000HP or so & causes Stop everytime so Power Armlets are a must.

The best way to beat it is just to use physical attacks & keep buffing your characters with Bubble,Protectaga,Shellaga & Hastaga. Make sure to have Curaja or Renew for curing setting these to a gambit would be a good idea. Now all you gotta do is find him.