And itll still be Luffy in the end lol. Nobody else is fully defeating an Admiral before he does. Same with a YC and a Yonko. Hell, its probably the same with a Gorosei, or the Gorosei doesnt lose here.
Either he defeats Kizaru, or he defeats Kizaru and Saturn (probably with the Robot) together. This is (unfortunately), the Luffy show, so the whole idea of Nika will be centered on him and not his crew. This is why the VA exist, for some of them to fight the weaker crew members when its time.