Poseidon

Poseidon is a mythical figure in Weyard’s past and serves as a boss in Golden Sun: The Lost Age. Among boss monsters he serves a notable influence upon plot progression. He resembles the classic image of king Triton.

Story
Poseidon is an entity that has long slumbered within the depths of the Sea of Time, near Lemuria. According to NPCs in The Lost Age, Poseidon was once the "true rightful king of the seas" far in the past (perhaps Weyard's ancient past), and is apparently the "Sea God" that the Shrine of the Sea God in Indra is dedicated to, but he has now become a mindless monster that would cause trouble if he wakes up.

At the end of Golden Sun and the beginning of The Lost Age, Mercury Lighthouse and Venus Lighthouse have been lit. This is what causes Poseidon to awake, and he in turn generates the large tidal wave at the start of the second game.

He is defeated by Felix's party later on as they attempt to sail into Lemuria, by using the legendary weapon against him, the Trident of Ankohl.

As a boss
Poseidon blocks the way into Lemuria and ensures that you cannot enter the place until you’ve thoroughly explored and completed the entire Great Eastern Sea. Otherwise, you’d be able to enter Lemuria as soon as you first acquire the Lemurian Ship. Poseidon achieves this not because of being too tough a boss; it’s that in battle, no attack whatsoever can damage him because of an impenetrable force field that guards him. Only using the completed Trident of Ankohl as an item can remove the force field of Poseidon, after which it becomes a normal boss battle that can be completed. The Trident can only be completed through thorough journeying and event completion around the Great Eastern Sea. (On a related note, Poseidon can be encountered in the Monster Battle mode, but fortunately he does not possess this invincibility.)

With the Trident in tow, Poseidon becomes a normal boss, and he has the following statistics: 4905 Hit Points, 162 PP, 302 Attack, 100 Defense, 185 Agility, 47 Luck, and he can act twice per turn. His resistances are highly lopsided; he is very resistant to Mercury, resistant to Venus and Jupiter, and very weak to Mars. When felled, the party nets 2930 Experience, 3762 Coins, and is guaranteed to receive a Psy Crystal.

Poseidon’s inventory of commands include a physical attack, the Ice Missile and Deluge Psynergies for moderate Mercury damage, and Ply Well for HP restoration. He has four monster skills: Counter-rush, which causes Poseidon to automatically react with a physical attack whenever he is damaged that turn; Typhoon Blow, which resembles Poseidon launching lots of energy bubbles from his trident and does strong Mercury damage to some Adepts; Watery Grave, which resembles Poseidon summoning a large 3D wave to wash over the party and does somewhat less damage then Typhoon Blow but strikes more Adepts and has a small chance of instantly killing them; and Ocean Fist, resembling an Adept getting caught in a fearsome tornado of water and energy, which may reduce that Adept’s HP to one but otherwise does small Mercury damage.

Battling strategy
Jenna learns Serpent Fume at level 26, so training the party to that level may make this battle easier. Also, if all six possible Venus Djinn have been obtained at this point in the story, Felix can use the immensely powerful Odyssey in battle, the only disadvantage being that it doesn't capitalize on Poseidon's weakness to Mars-based attacks. Other ways to take advantage of this weakness would be to utilize the Pierrot's Juggle (Mars-based) abilities, or redistributing one or two Mars Djinn to each character, which may permit some unique strategies.