Kraken

The Kraken is a unique boss monster fought in Golden Sun via mandatory encounter. This impressively sized and detailed squid-like monster is fought as the last of the four separate scripted monster encounters that occur during the voyage between Kalay Port and Tolbi Port across the Karagol Sea upon the Tolbi-Bound Ship.

Story
The Kraken is chief among the monsters that emerge within the Karagol Sea after the eruption of Mt. Aleph and the activation of Mercury Lighthouse, and these monsters have until that point never been sighted in the Karagol before. Prior to Isaac's wild ride on the Tolbi-Bound Ship (but apparently after the ferry service transports Saturos' group of Adepts and hostages across the sea to Tolbi), the Kraken and other lesser sea monsters terrorize the ferry service between Kalay and Tolbi Ports on either end of the Karagol Sea, even going so far as to sink and destroy one of the vessels and claim its unfortunate passengers. This effectively grounds the entire ferry service, to the chagrin of many a tourist because this is occuring during the season when the annual Colosso tournament is held at Tolbi.

When Isaac's party of Adepts, in their pursuit of Saturos, is able to elicit a voyage on the Tolbi-Bound Ship by agreeing to protect the ship from sea monsters as warriors, the turbulent ride that follows sees the ship attacked three separate times by minor sea monsters and knocking out three of the ship's oarsmen. Close to the end of the voyage, the Kraken climbs onto the front of the ship and attacks the ship itself, knocking aside the warriors Sean and Ouranos, (somehow) injuring a fourth oarsman, and keeping Kaja at bay. Isaac's group emerges from belowdecks to engage in an intense battle with the beast, eventually slaying it. With this, they save the lives of everyone on board and the ship arrives at its intended destination (or Crossbone Isle, depending on which four rowers Isaac chooses throughout the episode), but the ship has been stalled for time long enough that none of the warriors on the ship make it in time to register for the Colosso competitions (though Isaac's party is able to enter through other unique circumstances). Afterwards, even though the Kraken is gone, when Colosso is over none of the tourists from Kalay can return there because Kaja and the Tolbi-Bound Ship captain are too wary of the Karagol's dangers.

As a boss
The Kraken, as previously stated, is fought as the last (and obviously most noteworthy) of the four separate scripted monster encounters that occur during the episode on the Tolbi-Bound Ship. Defeating it is mandatory to progress through the main game, and once the boss is defeated the party gains 711 EXP, 5200 Coins, and a Water of Life.

The Kraken features 2400 HP, which is markedly higher than the previous main boss the player fights in the game, the Manticore's 1700 HP. It also has 46 PP, 216 Attack, 60 Defense, 80 Agility, and 21 Luck. As it is a boss, it can act twice per turn, which is quite dangerous considering its commands. It can physically attack, and it can cast the Drench and Froth Sphere Psynergies for moderate Mercury damage and cast Ply to heal itself by around 115 HP. Its somewhat low PP meter restricts it from using these Psynergy spells over a total of five or six castings, but its four monster skills are perhaps even more dangerous and don't run out: Water Blessing, a water breath attack for somewhat strong Mercury damage to multiple damage; Dark Blessing, a thick inky breath that does damage to multiple Adepts and may inflict Delusion on to any of them; Poison Beat, a green tentacle slap that damages one Adept and may inflict standard Poison; and Spinning Beat, a purple tentacle slap that does especially strong damage to one Adept.

The Kraken has a very strong Mercury resistance rating as might be expected of a monster hailing from the sea, and it has good resistances against Venus and Jupiter attacks and Psynergy as well, but it has very low resistance to Mars. To put this into perspective, summoning Mercury does ~75 damage, summoning Venus and Jupiter does ~110 damage, and summoning Mars does ~150 damage, and each Level 4 summon does roughly five times the damage of the Level 1 summon in its respective element. If you apply the rush-summoning tactic of summoning all four Level 4 summons on the first turn, the Kraken will receive about 2000 damage total, but your party will be very vulnerable to its repeated assaults and may all be downed before they can fell the Kraken unless the party is at a rather high level to endure it.

For those who do not favor rush summoning, a solid strategy is to have Garet attack with Heat Wave each turn, which can do around 130 damage, and have Mia use Wish every turn as well, while Ivan empties out his PP meter on Storm Ray and Resist and Isaac attacks with Ragnarok. (If you have the Vulcan Axe from Lamakan Desert, an alternative to Ragnarok is to equip Isaac with this weapon for this battle because this weapon's Unleash effect and even its normal attacks are of the Mars element, which capitalizes on the Kraken's low Mars resistance. Isaac unleashing its Barrage effect and Garet using Heat Wave each turn is a solid and effective damage plan to use for the whole battle.) If Isaac and/or Garet are in the Savage class, then Planet Diver becomes another good option--especially if they both use it.