I couldn't finish 'em. Looks like this's gonna get complicated.
Denn is the first boss in the game to demonstrate any kind of involved mechanics. At 80%, 60%, and 40% HP, the unnatural ripples around the room will start bubbling - first only two of them, then three, and finally all four. Players can interact with the ripples to disable them. If they are not disabled in time, a Baleen Guard will spawn. The Baleen Guards have low HP (roughly 400) and can be killed quickly, but it is important that the tank aggro them immediately before they start harassing the healer and DPS.
Beyond this, Denn uses True Thrust as a moderate damage spike and Hydroball, a frontal cone AoE that will damage and silence anyone it hits. In addition, at the aforementioned HP intervals he will use Jumping Thrust, a large damage spike that can leave the tank and healer on their heels right before adds potentially spawn.
The encounter can be fairly trivial, if slow, if the party focuses on disabling the unnatural ripples, preventing any Baleen Guards from spawning. However, if the party is feeling confident they can choose to ignore the ripples and subsequent Baleen Guards to focus on burning down the boss, as they will despawn upon his defeat. It is important that the tank and healer are prepared for this, as up to 9 Baleen Guards can spawn over the course of the encounter, which can very quickly get out of hand.