Star Magician

Star Magician is an optional boss monster in Golden Sun: The Lost Age. He resides in the deepest cavern of Treasure Isle, and guards the Azul summon.

The fight starts with one of each of the following "ball" monsters on his side: Guardian Ball, which either defends or casts Guard Aura on Star Magician, causing him to take 95% less damage; Refresh Ball, which casts Earnest Ply on Star Magician to heal him for around 1120 damage; Anger Ball, which will either attack regularly or explode upon the party doing very high damage; Thunder Ball, which casts some level 2 Jupiter Psynergies on the party and also recovers 10 PP per turn.

Star Magician himself acts thrice per turn, can cast Spark Plasma, Megacool, Mine Ball, attack regularly (which causes an orb to shoot out of his stave and collide with you instead of the usual whacking animation), and use Mystic Call to summon a random ball on the field. He is resistant to Mercury, but weak against Mars.

Some believe Star Magician is among the most irritating of foes in role-playing games, based on the power of both his attacks and the abilities of his "ball" underlings, which he can summon at will.

Strategies
For a moderate level party, summoning is the way to go here. While Star Magician is weak to Mars, Daedalus is not a good idea as he can summon a guardian ball on the turn after the first missile is fired, which can promptly Guard Aura the magician so the big missile does jack squat (to the magician himself anyways). Use Meteor to defeat him.

Do not use summons (or any other attacks, for that matter) on Star Magician while any Guardian Balls are present - they will more likely than not cast Guard Aura, which basically removes 95% of the damage you were planning to inflict. Guardian Balls should be taken out ASAP. When there are no Guardian Balls, take out Refresh Balls if they're present, since Earnest Ply is still quite annoying (though not as much as Guard Aura). Thunder Balls and Anger Balls can be safely ignored - all they can do is damage you, and they don't do their jobs well enough to be of concern to you, as long as your level and/or your equipment is good enough.

As Star Magician himself knows some powerful Psynergies, and you will probably end up with more than one Thunder Ball on the other side (whose damages can still add up), defence is important as well. Star Magician does not cast Break, so unleash Steam and Breeze as soon as possible to max out your resist bonus - this will greatly soften the blows, as the big guns from the other side are all psynergy based. Furthermore, have two people with Wish ready, so you can quickly heal when you need to. The other two people should be Garet and Jenna with their Mars Djinn, and they would be the attacker by unleashing Djinnis and summoning Meteors (again, don't bother with Daedalus).

An alternative to defeating the Star Magician is by destroying the Guardian Ball and Refresh Ball. One these are out of the way, then focus your attacks only on the Star Magician. If the Star Magician summons other Guardian and Refresh Balls, destroy them immedately. Ignore Thunder Balls and Anger Balls. Eventually, the Star Magician can only call up to four Thunder Balls. This will prevent the boss from calling monsters that can heal or give protection. Continune to focus attacks on the Magician until he dies. Avoid using summons or Psynergy that attack groups until the Star Magician is destroyed. Attacking the group would only give the boss another chance to use Mystic Call and bring up another Ball. After defeating the main boss, you can now focus on cleaning up the remaining monsters.