There are clumps of sargassum around the entire lair. They tend to be found at the edges of the water. Be careful collecting them, as zombies often hide under the ground and appear when it is least convenient.
Objective: Set the sargassum on fire in a barrel.
There are multiple barrels around the lair, usually near the edges of the coast next to the sea. Use the barrel then move away quickly, as a cloud of poison gas will be released.