Luffy is not beating any admrial there no need for it in the story .
There not the bad guys in the story even Akainu nice compare to the CDs and luffy will most likly not fight him .
I think Sabo defeating Akainu is a possibility. That said, I think the pivotal admiral situation will center around Akainu killing his daughter. Most authors strive for parallels. Oda is a stream of consciousness writer, thus, parallels are not a high priority, so my thinking may be off-base.
First, Akainu was hellbent on ending the lives of the children of Roger and Dragon. More recently, we have the scene where Akainu commented about how/why Kuma had no reason for living when Kuma endured Akainu's attacks and then escaped with ease to go help Bonney. Shortly thereafter, we have story elements with Akainu's daughter (newly introduced?) vs. BB's crew. I think there is a decent probability of the Gorosei or BB's crew facilitating/tricking Akainu into landing a fatal hit to his daughter and going insane and then destroying himself.
@AkainuTheGrimReaper, et al., thoughts? Approach it from a storytelling perspective, not power-scaling.