If this is really legit.
I'm saying long ago that Imu and Gorosei look like one entity during the encounter with Sabo in the Flashback.
Due to that I'm always have a feeling you can't kill Gorosei without Imu, since All Gorosei and their powers are a part of him, Imu need to be killed.
It seems Luffy fights Imu, while his crew and other allies fight Gorosei, they'll need to keep holding them until Luffy finishes off Imu