Kraken

One would think that a boss that can't attack would be easy, but the Kraken proves otherwise.

This boss encounter comes with 6 enormous KrakenTentacle enemies, all of which have Invulnerable as a permanent status effect which prevents them from taking any damage. Not only does this mean they'll be getting free hits on you, but they'll also block your movement. If you get surrounded by them there's no fighting your way out.

Don't think you can stay away from them on land either, the whole field will be flooded soon enough. They have LargeWaterTrail onMove, converting tiles to water and hindering any of your non-aquatic units by getting them Soaked. Fortunately they don't have any movement, relying entirely on Dive to get around, which means they can only move once every two turns. If you have passive shadow spread you could probably keep the water at bay. Otherwise you could try converting the field to lava, which isn't affected by LargeWaterTrail. Even if you don't fight over the terrain, the KrakenTentacles also get Soaked, limiting their damage as well.

However they also have StrongPheromones onAttack and Constrict, forcing you to keep an eye on your Lust as well as your health. Arguably this is what really makes them dangerous. Even if it isn't much per hit, there's 6 of them raising Lust with every attack. Fortunately their mana pool is small and Constrict also has a 2 turn cooldown, so if you can survive long enough for them to run out of mana you'll see their Lust output plummet. That's not recommended however, as the Kraken can use RevealTentacles to replace them with fresh ones.

As for the Kraken itself, besides renewing the summons, it has the WaterFall spell for area damage and creating water, and plenty of mana for it too. However for the most part it will defend and let its KrakenTentacles do the work, making it even harder to lower its massive health pool. The BreakerSword is recommended here, as are sources of electric damage.

Upon defeating the Kraken you can choose to harvest its tentacles to acquire 6 Tentacle weapons, which are mostly about Lust, or you can choose to watch and raise the max morale of your party.

It should be noted that any boss fusion or offspring lose the Giant trait, so if you wish to obtain a boss in all its glory you need to capture them.