Pirates of the Wild Sea – "Adventures of Captain Hack" [Campaign 8 – Mission 1]

Good afternoon! Could you please tell me how to defeat the Thunderbird? And what determines the size of the army given for the battle with it? Because I come with a large army, but they give me a very small one for the battle. I reduce its health to about 4500, and it still defeats me. I've already tried all the options, but I can't win.