The Serpent boss, Jörmungandr, is literally just an oversized snake. He has no additional special abilities other than improved attack damage and health that all bosses receive. Furthermore, Poisonous is often inconsequential to have on a boss, since you will often be out of blackspace. Furthermore, you would be allocating additional resources (like Health Potion) to kill the boss anyway. This makes him one of the easiest bosses in the game.