User blog comment:B214/Volume 22 spoilers/@comment-30048084-20161031183100/@comment-10330822-20161112224849

Well, we just assume that she just has normal Vampire powers, she may have other abilities well-suited for a Rook, we just don't know yet. Besides, you have Rooks that should be Bishops all over the place, just look at Koneko and Rossweisse. Koneko presumably only really has that kind of Rook-level strength "because" she is a Rook. Nekomata aren't said to be very physical strong, elsewise, her Senjutsu and Yonjutsu and other stuff are more Bishop-based, like Kuroka anyways. The fact that her biggest contributions to the latest half of DxD has been "sealing" should make that obvious.

Likewise, Rossweisse mostly uses magic, but Issei says that because she is a Rook, its like she is a "magic tank". So that Rook just made her magic even stronger, to the point she is like a cannon when she is firing magic.

Either way, I think its far too early so say whether Elmenhilde is really unsuited for being a Rook, we just don't know enough about her. Besides, she was acting as a vanguard, which means she must be very powerful if its in the traditional vanguard sense.