User blog comment:B214/Volume 21 info & spoilers/@comment-83.7.233.159-20160420144354/@comment-24.113.129.70-20160423224538

I didn't mean to imply that her proficiency in magic was the only thing Rossweisse had going for her, she has enough power to take on even Ultimate-Class Devils. Her ability to use magic spells at a fraction of their usual cost only supplements her capabilities, allowing her to conserve and consolidate her power so she doesn't "run out of steam" too quickly (which is a problem a lot of magicians face).