Unlike the Kraken from the rare raid, the Krakens in this fight are quite resistant to magic, and they hit harder than the raid versions. Kraken uses magic attacks and has an attack that can inflict blind. After the first Kraken reaches a certain amount of HP, an even higher HP Kraken will spawn on the opposite side.
3 starring this fight is pretty easy since there is no attack/elemental chaining required. Simply beat the boss with all units alive without hiring a companion.
Easily doable with the thunder missile units. Coeurl esper fits both of them better since it has missile attack, but Ramuh is still a pretty decent boost since he has lightning attack increase with aquatic and water killers.
Magic tanks aren't really that necessary here, but they can help prevent Kraken from hitting your main damage dealers with a blinding attack. Fenrir is a pretty easy way of reducing damage your entire team takes, so you can bring any support unit and slap them with a Fenrir VC.
A safe unit to bring is a healer that also has Esuna skill to remove blinds. An even safer unit is if your healer can also revive, to make sure that you get the "no deaths" mission.