This would have been the best fight tbh:
Killingham with his nightmare manifesting powers would have been perfect for Usopp too but he tends to fight female characters with hax powers that only he can outsmart and overcome and Killingham isn't female obviously. The SHs couldn't even handle Gunko and Usopp stands zero chance against Killingham too so idk.
Since Luffy and Zoro are the only ones with Adv CoC too, nobody else can defeat the holy knights too from the SHs so I'm not sure how the SHs are meant to face and defeat them, never mind anybody else who lacks ACoC too, including the Revolutionaries.
Oda ironically tried to replace Logias and wrote something much more problematic for himself with the HKs. Don't even get me started on Awakened Zoans too lol but god help us all if Logias actually get awakenings too as they should.
People who think usopp will beat/fight killingham himself are in rude awakening
He will need a 20 year timeskip or something. He's 100 years too early to challenge and beat a Holy Knight /s.
Kinda ironic I use a Shanks related example I guess lol.