Golden Sun: The Lost Age/Mars Lighthouse, Part 2

Ultimately you don't need to worry about getting Iris. Dullahan is borderline impossible and even with this guide's help he's still very strong. Daedalus, Catastrophe and Azul will be good enough; Charon and Iris are just bonuses, although Charon is not that difficult to get, by any means (unless you didn't update, in which case he's impossible to get).

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.

Final Boss: Doom Dragon
The Doom Dragon is your final opponent. He has the same amount of HP as Dullahan, although is significantly easier (yet still difficult). For ever third of its HP you knock off, it loses a head, and the battle gets harder.

Three Heads Intact
In this phase, Doom Dragon gets 4 attacks a turn (that's right, one for each of your attacks), although the most this will be is an assortment of weak special attacks or level 3 Earth and Fire Psynergies like Stone Spire, Quake Sphere, Inferno, and so on. 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 annoying is Djinn Blast, which puts all of one character's Djinn on standby - basically a weaker version of Dullahan's Djinn Storm.

Megiddo from the Sol Blade as well as unleashing attack Djinn and Aura/Wish Psynergies will pass the time until you can reel off summons.

Two Heads Intact
Now 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 does between 300-600 damage to every party member. If a Djinn Blast has occured 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. You might want to consider stopping the relentless pounding with Megiddo to heal.

One Head Intact
Uh oh. Doom Dragon only gets 2 attacks a turn now, but Cruel Ruin can sometimes be used 3/4 turns in a row, and to make matters worse, your foe gets Djinn Storm now. Djinn Storm followed by Cruel Ruin is pretty much an instant lose. If your backup party will be of any use during this battle, it's now.