Luffy might have finally gained the Power Up that allows him to fight Peak Top Tiers, but Oda already showed us how this Sun God Nika is countered. Gorosei even clearly said it themselves "Sun God Nika is ONLY LIMITED by it's own IMAGINATION". So those who can temper with Luffy's Mind can definitely Nerf him & allow them to beat him. That's why Imu-Sama was given such Eyes, this way even Sun God won't be enough against him.
I said this Last Year & it's finally confirmed, the Fact that Oda made Luffy's Peak be based on his Imagination, means those who are supposed to still be a Threat for him are able to Nerf his Imagination, They can attack your "Mind" directly
That's what Mihawk did against Luffy in Marineford
That's how Shanks makes his Opponents unable to focus & use their CoO/Future Sight
That's how Saturn is attacking People's Minds
I will repeat what i said recently, Oden is a "Great-Grade" Master Swordsman, and Kaidou had Comparable Haki, Luffy splitting Sky with Kaidou, and Zoro giving him a Scar + Taming Enma indicates that both reached that Level of Haki
But there is still a Higher Level that isn't revealed yet, "Supreme-Grade"
Only Supreme-Grade Blades Wielders & their Rivals have such Level
Mihawk
Roger
Shanks
Whitebeard
Gorosei (One of them carries a Supreme Blade)
Rayleigh
Garling
Imu
Garp
Sengoku
Xebec
... etc
Kaidou & Big Mom are only able to Catch up thanks to their DFs + Durability
While Blackbeard have Two DFs (With one of them Countering other DFs)