Yeah many people is saying he could use the serum in Armin but I think he’s too far from them in distance? He’s outside while the squad is on the other side of the wall. Not to mention his horse is gone and he’s actually sorrounded by titans. And even if he’s lucky and gets to kill those titans and survives, he’ll probably end up using a lot of gas too. On the other hand, if by some lucky chance he got the oportunity to use the serum on Armin, that would mean titan!Armin would have to eat someone in order to go back to being human again. So that would mean anyone from Levi’s squad dying instead. I don’t think Levi has many chances to be honest. He could always use the serum on himself as a last resort, and eat a random soldier to go back to being human, but…