Serpent

The Serpent is a boss in Golden Sun: The Lost Age located in Gaia Rock on the island of Izumo. It resembles a long, green, snake-like dragon and is associated with the Venus element.

Battle stats
The Serpent has 3536 Hit Points which regenerate at varying amounts, 160 Psynergy Points, 249 Attack, 76 Defense, 135 Agility, and 46 Luck. It is most resistant to Venus-based attacks, equally resistant to Mercury and Mars attacks, and is notably vulnerable to Jupiter attacks. When slain, the Serpent yields 1995 EXP and 2898 coins and, while it isn't dropped in battle, the Cloud Brand can be picked up after Susa leaves the chamber.

What makes the Serpent unique among bosses is due partly to where it's battled: The Serpent's chamber is deep within Gaia Rock, where sunlight cannot reach...normally. By exploring Gaia Rock and solving its puzzles, anywhere from one to four beams of light can reach the Serpent, significantly weakening it. The Serpent can normally attack three times per turn, but if all four lights are lit it can only attack twice. Also, the amount of HP it recovers each turn depends on how many lights enter the chamber: If no more than one or two lights are lit, the Serpent is effectively invincible.
 * 0 lights=2430 HP
 * 1 light=810 HP
 * 2 lights=270 HP
 * 3 lights=90 HP
 * 4 lights=30 HP

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.