@Yamamoto10320 what is he doing??
Luffy fought with Kizaru one time and they were equal. Maybe you could argue that Luffy was a bit stronger.
The second time ( the pizza fight I mean) was not actually a fight. Kizaru killed VB and luffy cought him . As simple as that.
Also this is a free for all. It isn't a 1v1. Luffy's stamina issues put him in major disadvantage here.
So it is inevitable that he will be the weakest in this situation.