You just have to be level 47 or above, as that’s the quest level for Guilding the Bilious, have completed its pre-requisite Shadows Uncast (which you had to anyway to get the previous GC promotion), and be at Chief Sergeant rank. How do you get a second serpent lieutenant?
You’ll be granted the rank upgrade immediately upon successful completion.
Your adventurer squadron needs to successfully clear the level 40 checkpoint mission. How do you rank up first flame lieutenant? If you change your Grand Company you will still keep your current Adventurer Squadron members but will need to obtain Second Lieutenant rank with the new Grand Company before being able to do any actions with them. To unlock Adventurer Squadrons, players must obtain Second Lieutenant rank with their Grand Company.