Who do you think will fight Saturn?
Group effort. Luffy, Zoro, and Sanji, along with Kuma and the Ancient Robot definitely, with Dragon probably showing up too, and the Seraphim being possible fighters as well.
Luffy fought a lot the day before this, and has now used One Gear 5 after getting kicked through a robot and going through the laser grid twice, once in G4 and once in G5. Dude can't hard-carry against Saturn, especially since Kizaru is down, not out.
Kizaru was focused on trying to kill Vegapunk, now, once he gets up, he can focus on actual fighting, which will make him a lot more dangerous (unless he betrays Saturn).
Sanji unlocks CoC and uses it to knock out the Marines targeting Bonney, Zoro pays Bonney back for keeping him from killing Charlos in Sabaody by cutting Saturn's arm off and freeing Bonney.
Ancient Robot fights Saturn, Kuma gives Saturn a pain-bubble that would make Zoro blush, maybe even some S-Snake S-Bear protecting Luffy and Bonney from Saturn.
Dragon shows up to and finally does something.
Saturn is a Raid boss that shows up in the Campaign of a game before you challenge him fully in his own lair (Mary Geoise).
They are not beating Saturn. They need to GET THE FUCK OUT OF DODGE.