Golden Sun/Venus Lighthouse Aerie

After dropping down the hole, an elevator will carry the party to the aerie of Venus Lighthouse. Here you'll overhear Saturos and Menardi arguing with Felix and Sheba. Eventually you'll enter battle with Saturos and Menardi.

First Battle
Focusing on Menardi first would be recommended, as she has the Death Scythe ability that not only is quite damaging, but also could potentially insta-kill one of your allies. Also, she has less HP (2600) than Saturos (3000).

Saturos will be the one doing most of the damage and other status effects. He will also cancel status buffs, so increases won't do much.

You should spam multi-attack Mercury Psynergy and/or Mercury-based Summons to quickly take care of this battle. Keep in mind recovery time for the next one.

If you have the Gaia Blade, now is a good time to have Isaac spam attacks.

Defeat them and another cutscene will follow. The Venus beacon is lit, Felix and Sheba leave, and Saturos and Menardi merge to form the Fusion Dragon, attacking the party.

Fusion Dragon
Despite being just 1 monster, Fusion Dragon will have 2 attacks. It has a very devastating attack called Outer Space, which behaves similar to a Meteor Summon. Feel free to unleash all your summons in battle, Wish yourself to full HP, and win.

Epilogue
Once the dragon is defeated, another cutscene leaves Isaac and friends back in Lalivero. They resolve to seek out Jenna and find Lemuria, and receive the Black Orb needed to pilot Babi's ship. Enter the guarded building in the northwest corner of Lalivero town and walk to the end of the gangway. Use the Black Orb to access the Lemurian Ship.

To Be Continued in Golden Sun: The Lost Age. The Lost Age manual will tell you how to get the passcode from this game to The Lost Age for transfer.