That's our Ulti!!!
She went toe to toe with the "5th emperor" on a CoA clash! Damaged Luffy in the process. Had Luffy pinned and Luffy was about to go gear 4th just for Ulti. Just think about that... Even Luffy acknowledged she was strong right to her face.
He is the fifth emperor the same way buggy is a warlord.
I said it contradicts what Zoro told Luffy at PH to get serious. Luffy is not the type to get serious. He himself said he underestimated Ulti.
I didn't see Kidd or Killer panting. I also don't think Kidd or Killer need a sneak attack to defeat them. They just make good use of opportunity when they are in territory with 2 yonko. They would fight Kaido so taking as little damage as possible would be helpful.
I gave you a possible reason, also even when zoro told luffy to get serious he was only using 2 swords and what zoro means by serious is not going all out and exhausting himself he means not fucking around like what luffy did on punk hazard.
Being serious =not fucking around
Being serious=/=going all out against every opponent.
The context of the quote matters, luffy wasn't just holding back he was acting like pre timeskip where it was an adventure and not a serious fight.