The only reason Luffy was able to land Bajrang Gun is because Ryou with AdvCoC need not make contact. Which bypassed the barrier and overcame the haki shield of Kaido. If Kaido used haki, there is every chance that Akainus fistwouldnt touch him, then Akainu would need greater haki to overwhelm him. The great tiers you say were unable to use the haki barrier. Ace had to save Luffy so he didn't have time, Jimbei didn't have the haki to overpower Akainu and WB had less time and no reason to stop because he was dying. So if the top tiers use haki, Akainu and his muscular force can be stopped.