So far in the game, I usually start a boss fight by using support-type Djinn to boost the entire party's defenses, attack power, and speed. Afterwards, I use attack-type Djinn like Flint and Fever to attack the boss and continue to set up for summons. If I use nothing but Djinn on all of my characters, I can use the third level summons (the strongest available right now) on turn four. At that point, I have all four party members use their summon spells to attack the boss and boost their elemental power. However, the boss typically dies after only the second summon attack, leaving me disappointed with the battle's anticlimactic end.
Even if I mix up my strategy and have some characters do nothing but use Psynergy spells to heal or attack, or focus on using lower level summons, the boss is typically dead by the fifth round of combat. Every time, it is disappointing. Four or five rounds of combat is not enough time for Golden Sun's combat system to fully shine. Where I am in the game, each character has three or four Djinn, more than half a dozen Psynergy spells, summon spells of various power levels, physical attacks, and an inventory full of recovery items. If each character can only act four or five times in the whole combat, the player has no chance to really use these options.
The most annoying aspect of all of this is that it is only after a full barrage of summons that the gameplay really gets interesting. In the aftermath of a high level summon attack, a character gains a large increase in elemental power, but is suffering from severe stat loss due to the used up Djinn. At that point, the player has to adapt his strategy. Since the characters' Djinn only come back one at a time, the player is forced to carefully choose between using the Djinn that are slowly coming back to get another chance to summon, or let them recover in order to restore the party members' stats. Tragically, I have never had the chance to go through this phase of a battle yet, thanks to the short length of the fights. I haven't even been able to play around with different strategies for this situation. It is frustrating.
The bosses in Golden Sun should have had at least double the number of hit points they currently possess. In my experience, boss battles do not suffer from being long. Rather, many of the best RPG boss fights are long and difficult ones, where the player is forced to pull out every trick and strategy at his disposal to come out on top. A really long boss fight would have been really enjoyable in Golden Sun, because of its uniquely dynamic combat engine.