How do you defeat the Star Magician?

Yes the Star Magician is extremely hard but not unbeatable. I recommend everyone in both parties should be over level 45 or 50 and each have a considerable amount of Djinn. If Mia knows Pure wish use that. Also I recommend you use powerful summons like Haures and Castrophe. It took me a lot of tries but if you get everything listed he should be easy.One last thing. Focus ALL your attacks on the Star Magician even though the balls are beating you down. When he is gone they are easy.

Hope I helped.

O.K. I admit that Star Magician is a long battle, but with the right strategy he is very easy. I suggest, like the previous entry, that you try to have high levels. Make sure you have all other summons besides the ones from the bosses. You might consider trying other bosses first, like Valukar, Sentinel, or Dullahan (you must have played and transfered data from Golden Sun to verse Dullahan). I have a guide book and it claims that the Star Magician is the hardest boss to beat, but I found it easy and long. My levels for Isaac's party were around 35 and Felix's were 41. My strategy was to put Jenna and Mia on healing duty. Send Isaac's party out first with all Djinn on standby. Use all of your best summons and keep this party alive as long as possible. For Felix's party try putting 4 Djinn on standby for each character, except for Jenna. Only use one of Jenna's Djinni-Spark-to revive others. When onto Felix's party use all high level summons first round and Jenna's Cool Aura. Continue to use Cool Aura every round (even if you think you don't need it), and use Sheba's Ether Djinni to re-fill Jenna's PP. As for Star Magician, you must attack any Recovery Balls (they heal 1000 hp), and Anger Balls (selfdestruct causing lots of damage). If either of those balls comes up destroy them immediatly. Thunder and Guardian Balls will bug you but don't worry about them. Make sure Gaurdian Balls don't last long though or the battle will last forever.

Priority List: 1) Star Magician
2) Recovery Balls
3) Anger Balls
4) Guardian Balls
5) Thunder Balls

Hint: Let 4 thunder balls get on the field and you'll win, because they suck, and all you will have to do is heal and attack Star Magician.
Hint #2: Star Magicians weakness is fire.

Good Moves: 1) Oddyssey
2) Cool Aura
3) Tempest / Destruct Ray
4) Inferno
5) Diamond Berg / Violent Cool

I now, thanks to you all, have an even better strategy myself:

Your party should be as follows (all with rightful djinn):
Felix (preferably with sol blade) - attacker
Isaac(preferably with excalibur) - attacker
Jenna(preferably with clothes that restore pp each turn)
Mia(preferably with clothes that restore pp each turn (i ran out of pp with mia, then added clothes, it helps))

Turn #1: Felix attack refresh ball
Isaac attack guardian ball
Jenna heal (even if u havent lost hp, it helps anyway)
Mia heal/shade

Turn #2 (if the two balls you attacked are destroyed, if not, repeat turn #1 again)
attack any balls except thunder balls with Felix and Isaac

repeat process until star magician summons 4 thunder balls

Turn #Thunder Balls:
4 thunders are on the field, attack star magician with sol blade and excalibur, use NO DJINNI OR SUMMONS! heal with Jenna and Mia, even with full hp, if you don't maintain hp, you WILL lose, even with all thunder balls

keep repeating until star magician is dead, do not destroy any balls until he is gone, i beat him twice with this strategy (once in treasure isle, once in battle dome place type thing)

this is a flawless strategy, i know, i've never been flawed, you will not die if you follow these exact instructions, it is very similar to goldengod's one, but that is where this much better one is from, thanks goldengod, you really really helped to uncover this one, now if you'll excuse me, i'm off to submit this all over the net!