The optional fights before approaching the final boss helps tremendously. Again, they are not required. Once you approach this battle, you cannot go back, so do everything you can and complete whatever you can.
Anyway, return to Prox via Teleport, and then leave to the north and head back into Mars Lighthouse. Follow the same path you've walked many times to reach the torch room, and use the Teleport circle to ascend to the aerie. Hop around and approach the well. You can find out by yourself what happens here, but then a boss battle will occur. It's a good idea for your first party to have no standby Djinn and your second party just enough to use an assortment of powerful summons. Also, the Sol Blade should be equipped to whoever out of Felix and Issac is in your first party.
Final Boss: Doom Dragon
The Doom Dragon is your final opponent. He has three heads, which are technically fought as three separate battles in a row, with no rest in between. The first head has 5000 HP, the second has 4200 HP, and the third 4000 HP, giving a grand total of 13200 HP. Because they are separate battles, attempting to summon rush will not work because the HP that the summons depend on rely on the HP of the fight's phase, not the total. This boss is weak to Jupiter, so Excalibur's unleashes will do the most damage when maxed out.
With all three heads intact, Doom Dragon gets 4 attacks a turn (that's right, one for each of your attacks), although most of these will be an assortment of weak special attacks or level 3 Earth and Fire Psynergies like Stone Spire, Quake Sphere, Inferno, and so on. The most these will generally do is about 100 damage to everyone. He also has Earth Force, an attack which does the same thing as Petra or Ground, and Guard Aura, which is annoying. This is the same as if Doom Dragon had unleashed Shade or Flash, meaning even IRIS gets reduced to about 100-200 damage. But by far the most devastating is Djinn Blast, which puts all of one character's Djinn in recovery - basically a weaker version of Dullahan's Djinn Storm.
When Doom Dragon goes down to two heads, it gets a little harder. Doom Dragon now only gets 3 attacks a turn, but the Psynergies it uses get more ferocious and damaging. But now you have to contend with Doom Dragon's best attack, Cruel Ruin. Cruel Ruin usually does between 150-400 damage to every party member, but can actually do well over 800 damage. If a Djinn Blast has occurred sometime recently, you can expect some deaths to occur. Now you'll want to hide behind barrier Djinn and Haze as much as possible. Healing Psynergies and Djinn will become more necessary, and thankfully this still allows for summons. However, do bear in mind that constant summon preparation will mean your characters are not as strong as they would be normally. An interesting trick is to use
When it goes down to its final head, things get even more devasating. While Doom Dragon only gets 2 attacks a turn now, Cruel Ruin is used more often. To make matters worse, Djinn Blast gets replaced by Djinn Storm, which puts your all your active party's Djinn in recovery. Djinn Storm followed by Cruel Ruin is pretty much an instant party kill. If your backup party will be of any use during this battle, it's now. In addition to these Doom Dragon also gets a few new special moves, including Darksol Gasp, which is a relatively powerful move that inflicts Haunt. Since Doom Dragon at this phase can do 2 attacks, an interesting tactic is to use and alternate between Petra and Ground to ensure Doom Dragon has only 1 command to use and to avoid a potential Djinn Blast + Cruel Ruin combo.
This is why getting Iris helps you out so much. If your backup party is weaker than your first one, then even if you summon rush the Doom Dragon at the first opportunity you might not get rid of its last head quick enough. In which case, everyone will be on the verge of death. Iris will not only get your backup party back into the groove but it'll also resurrect all of your initial party. Now, if your backup party dies, you first party will step back in again. Although it's unlikely Doom Dragon will either manage to mow down the equivalent of three parties or the battle will last that long, your first party after being resurrected can build up Djinn for Iris and resurrect everyone again. Of course, the only thing that can stop this is Djinn Storm.
Once the Doom Dragon goes down, you'll get another scene.
The ending
Congratulations! You've completed Golden Sun: The Lost Age! There isn't really much after the Doom Dragon battle, except for a brief playable period with Felix and Issac where you can talk to the various townspeople of Prox who will thank you for what you've done. Oh, and quite a few scenes in the aerie of Mars Lighthouse, Prox and with Alex, especially where he went after Jupiter Lighthouse.
Once the ending's finished, you can choose where to save your clear game data. This is mainly for use in the Battle section of the game, accessible from the main menu. Also, with clear game data you can enter Easy or Hard mode of Golden Sun: The Lost Age.
Easy mode is where all your characters are at their old levels and stats (although they lose all their equipment). Basically this makes every enemy and boss a pushover. Hard mode is the opposite - all the enemies now do twice as much damage and have a lot more HP. If you really, really want to challenge yourself, and this really isn't worth it, you can play through the game again in Hard mode. Update along the way, and try beating Dullahan and the Doom Dragon at this difficulty. This time, Dullahan has 16,383 HP, while the Doom Dragon's 3 forms assume 7,500 HP, 6,300 HP, and 6,000 HP for a combined total of 19,800 HP.