User blog comment:Piyush Banerjee/Shin DxD 2 discussion/@comment-39.32.81.212-20181220010137/@comment-2601:87:8300:20D0:519:C459:6F48:48D0-20181220040128

In the novel one part says sisters, so its safe to assume it means Rias and Grayfia. This is a way for to surpass her thus giving a good chance that it was Rias who defeated Grayfia.