During the stage, Leviathan sometimes sends his tentacles to disable a tower. Unlike Vez'nan however, this ability hardly poses any threat, since Leviathan uses it very rarely, can only affect towers next to the shoreline and most of the time, when the enemies reach the tower, it has already worn off. After wave 15, Leviathan surfaces at the large pool of water at the northeastern corner and proceeds to the eastern exit.
Compared to his elite minions (Blacksurges, Bluegales and Bloodshells), Leviathan is laughably easy. He continues to disable towers, however, he must stop whenever he does so, making him more vulnerable to your towers; and that's all he's able to do: he has no more tricks, yet unlike Sarelgaz, he doesn't have extraordinary health to balance that out. The minions which spawn alongside him consist almost entirely of Greenfins and Deviltides (and an occasional Redspine), which can be dispatched very easily if you have managed to pass the 15 brutal rounds previously. When he gets near the exit, he'll break the pillars blocking his path and summon a large horde of Bluegales and Bloodshells, though that shouldn't happen because Leviathan is so weak and slow that he's more likely to die before reaching it.