The Star Magician is the Mercury counterpart of a series of 4 difficult bosses guarding powerful Combo Summon Tablets. Although the Magician, attacking twice a turn with powerful Mercury and Jupiter Psynergy like Spark Plasma or Freeze Prism, is not so hard, he summons Spheres (I refuse to calls them balls) to aid him. Besides attacking, each Sphere has a special ability:
Guardian: The green Spheres can project a shield around the SM to drastically
reduce the damage done to him for a turn.
Refresh: The blue Spheres can use a healing Psynergy called Earnest Ply on the
SM, which heals him for at least 1000 HP.
Anger: These red Spheres detonate themselves on your party with explosive
force, killing themselves but doing lots of damage to your party.
Thunder: The purple Spheres attack with level 2 Jupiter Psynergy like Shine
Plasma, Flash Bolt, and Storm Ray.
For once, don't even standby your Djinn before this battle. Have one party member keep your party healed while the others attack all the non-Thunder Spheres. You may also want to have two of them alternate barrier Djinn (like Flash and Granite), a third heal (preferably with the Iris or Mysterious Robe and Psychic Circlet so they can cast Pure Wish every turn), and the fourth attack. Though this is slower, it is virtually impossible to lose this way. Anyway, once you have gotten the SM to have 4 Thunder Spheres out (Anger Spheres will also do, but they may use Angry Bomb and be replaced with Refresh or Guardian Spheres), start attacking him. Don't hurt the Spheres, though. Although you will take a lot of damage from various Psynergies this way, the battle will still be much easier than it would be otherwise. Just keep your party healed and attack the SM (not his Spheres) until he goes down. A single turn of ferocious Psynergies should then take the Thunder Spheres down.
The brutish beast Valukar is the fire Guardian of the Combo Tablet Daedalus. He would be a pretty average Boss fight normally, but there are a few twists that make him tougher. For one thing, he has an attack that will likely stun it target and do damage. Make sure you have a way of curing stun on at least two characters before battling Valukar. His most unique ability, however, is his ability to use your own Djinn against you! That's right, if you leave Djinn on standby after your turn, he'll most likely use them to Summon on one of his two attacks. He also has an attack called Djinn Stun that puts the first Djinn of each of your party members on standby. Make sure to take advantage of this by Summoning the spirits yourself if he doesn't (get ready for fun if he uses Djinn Stun twice in a row). Other than that, just use the standard strategy for most Bosses and you should win.