Oda is intentional dragging out the Chapters so he can land at Elbaf at Ch.1100
Luffy without Acoc is Admiral Tier
Luffy With Acoc is Yonko Tier
Kizaru taking 0 damage thus far is intentional
Luffy using all his stamina by using his more taxing forms will result in him reverting back to base form.
Fatigued* Luffy In Base using Acoc Reduces Kizaru HP from 100% to 0% in a single strike Emphasis there's the Massive Gap Between Yonko and Admiral Status Similar to Whitebeard vs Akainu
Imo* The second Luffy Uses Acoc Kizaru will be defeated similar to Luffy vs Bellamy
Kizaru Dashing around at Lightspeed reflecting off Surfaces going in for a finishing Blow only to be knocked unconscious by a King Pistol*