Darkest Dungeon II

The lair boss in the Shroud is the Leviathan, a massive fishperson found at the Sacred Pier. I would argue that the Leviathan is one of the easier lair bosses. It has two main "phases" - the head of the creature that can do high stress and debuffs to the whole team, and its hand that periodically rises above the water to menace the heroes. The hand can grab 1 hero at a time with its Undertow ability and will hold them underwater indefinitely unless the hand or creature is killed. Therefore it's important that all members of the party be able to do some kind of damage, even DoT, to the first row where the hand is. Without that, the grabbed party member will stay down there for a long time, taking damage and unable to use abilities. This is similar to the hag's cauldron from the first game, for any OG DD players reading this.

The hand doesn't have a huge amount of hp, but removing a party member will put your heroes out of position (unless it grabs the back row) which could mess up their abilities. If you're getting close to this fight, I would recommend rearranging your party formation and abilities for flexibility. Another strategy is a high move resistance. The hand makes a move check before grabbing, and if your hero succeeds, they will not be dragged underwater. Without Undertow, the creature's danger is significantly reduced.

Just keep up the pressure on the hand and head, relieve stress and heal hp occasionally, and this fight will be over before you know it!