My biggest issue with Usopp fighting the Siblings is that if he has the AP to defeat one of them why didn't he use it in Dressrossa?
At least Nami actually defeated About 9 strong Characters (8 Big Mom Ministers and Orochi) and knocked out Human Form Chopper with her lightning. Usopp didn't couldn't take out Trebol who's weaker than either of Ulti or Page One