User blog comment:B214/Question and Answers 4/@comment-26216310-20150409072410/@comment-26209278-20150409091118

Well, she was a contestant to the position of Leviathan along with Serafall, so i'd say those two are pretty much as strong as each other, just to give an indication of Grayfia's strength.

As B214 said, she isn't as physically strong as Sirzechs, she's just more bossy/has female power over Sirzechs.