Some more Details that they dont mention in the Anime:
L'Arc and Glass are not full-fledged Cardinal Heroes like Naofumi, Motoyasu, Ren and Itsuki.
There is a system that prevents Cardinal Heroes from crossing into other worlds (Naofumi actually tryed to follow them in the Novels, but when he reached that "portal" he got a Warning that Cardinal Heros arent allowed to go to other Worlds), and they can only be given a temporary permission to cross by yet unknown "Guardians".
These two are called "vassal weapons" holders as they called themself in the Novels, their weapons have similar characteristics and can use skills like Cardinal hero weapons, but are weaker in the overall concept. Therese is a normal magician, although "Normal" is not quite right, her race was called in the Light Novel "Jewel" which made her a Jewel "Human"(which is why she had this great jewel on her forehead and she also speaks with her jewelry). She is a Member of L´Arcs Party like Raphtalia and Filo are in Naofumis Party.
Vassal Weapon Carriers can according to L´Arc easily cross the border to another world with their companions.
The Main Task of Vassal Weapon Holder as he also mentioned to Naofumi are to Cross the Border Between Worlds and try to attack enemy Cardinal Heroes and kill them, and also Supporting Cardinal Heroes (of there own World), while the Cardinal Heroes ( like Naofumi, Itsuki, Ren, Motoyasu) have there Main Task in Protecting the World against the Waves.
And the Reason why the Soul Eater Shield is so effective against Glass is, that she from is from a Race called " Soul Person".
The Soul Eater Shield is normaly only effective against Heroes ( who use SP=Skill Points, which they need to use Weapon Skills) and Ghost- Like Creatures which he can kill with the Counter Attack.
But because of her race, Glass uses a status called Soul Power as her power source. Soul Power is an attribute that constitutes the Sprite People's Vitality, Magic, Spirit, and Level. The more Soul Power she has, the more powerful she is, as Soul Power also influences her Attack and Defense. She can execute skills by using Soul Power. If her Soul Power is low, she will get weaker.