Serpent

The Serpent is a boss in Golden Sun: The Lost Age located in Gaia Rock (known to the residents of Izumo as Mt. Mikage). It resembles a long, green, snake-like dragon and is associated with the Venus element.

Battle stats
The Serpent's statuses vary, depending on how many lights are lit in the dungeon prior to engaging it, the following are its statuses when all four lights are lit. It has 3536 Hit Points, which regenerate 30 HP per turn, 160 PP, 249 Attack, 76 Defense, 135 Agility, 46 Luck and the ability to attack twice per turn. If none or only a few of the lights were lit, it will recover enough HP to effectively make it invincible and attack three times per turn.

It is most resistant to Venus-based attacks and in decreasing order to Mercury, Mars, and finally Jupiter. When slain, the Serpent yields 1995 EXP and 2898 coins.

As well as being able to attack physically, the Serpent has several powerful abilities; the psynergies Quake Sphere and Briar, for Venus damage to multiple targets and Cure Well to restore roughly 150 HP (300 if used it twice on the same turn). Its monster skills include Black Ice and Sand Breath for Venus damage to multiple targets, Toxic Breath for damage to multiple targets with the possibility of slapping them with poison, and Mighty Press, where the serpent crushes a target for heavy damage. The serpent is easily one of The Lost Age's most challenging storyline bosses, even when weakened.

Battle strategy
If you didn't weaken it with all the light beams, watch with horror as it recovers all its health after your inital summon rush. Oh dear. But thankfully, you did use all four light beams, so it only recovers 30HP a turn (this will only slow you down a bit when you're resorting to Psynergies and attacks).

So, the first turn goes Judgement, unleash Cannon, Thor, Boreas, and the next turn should go Ragnarok, Meteor, unleash Blitz, unleash Shade. You may be in a bit of a pickle by this point, especially since Toxic Breath can inflict Poison and a fair amount of damage, and the fact that the Serpent attacks twice in one turn.

Unless you need to do any drastic healing, the next turn should go: unleash Flower, Serpent Fume, Shine Plasma, Cure Poison (assuming there is somebody poisoned, otherwise use Diamond Dust or unleash an attack Djinni). If anyone is downed, use a Water of Life (these are best put on either Piers or Felix) or Jenna's Spark. Turn four should go unleash Steel, unleash Spark (if someone's down, otherwise Serpent Fume), Shine Plasma, unleash Chill. The Serpent shouldn't survive this.

Cultural reference
The serpent bears a great resemblance to the Japanese serpent-dragon, Yamata no Orochi, which was defeated by the god Susano-o in order to free the princess Kushinada. The Cloud Brand found near its tail is the same as the legendary sword Kusanagi found in the Orochi's tail.