If adv conqueror is the way to beat them then the godknights got absolutely destroyed on GV. Potentially the gorosei too. I wouldn't be surprised if Rocks was able to beat Imu in his prime but Garp and Roger stopped him because they couldn't handle seeing slaves enjoying freedom. 
Only half joking.
The way Oda already framed Xebec as a team effort and shrouded what exactly happened to him implying Roger/Garp did not just kill him and God Valley as a whole being erased from the map? I think there is actually a pretty decent chance that Imu just pulled up and erased both Xebec and God Valley/put Xebec in some cryo-pod as a vessel for later.
