According to the reports of the mark on his arm, the shell is made of metal. It is a thick, highly durable metal that can heal itself. But the metal is not as hard as diamond, so he is sure that he can cut it.
The reason why someone as weak as him can breach the shell while the librarian and wayfarer can't is because each wayfarer has a defensive magnetic field around them that protects them from the winds of the planet and from each other's magnetic control.
So they can't do much against each other. As long as a wayfarer is alive, unless it is very weak, it will be able to resist another city with the help of its librarian.
As for breaking through the shell, he is not the only one in the squad who can do it. In fact, Command gave them a powerful tool to breach the shells of wayfarers. That powerful tool is acid.
