It depends on the match-up. GB defeated King and Queen quicker than Aokiji defeated Jozu at MF. He almost had the Scabbards and Yamato at his mercy faster than Kaido, someone stronger than Akainu. GB controls the battlefield, so it could be harder for Law to fight GB than Akainu, who's a straightforward fighter.
Besides, I don't get why Law has to be a mid-dif fight for Akainu. One attack is all he needs to end Law's life, unlike Big Mom or even BB for that matter. And Akainu isn't a level or two above the other admirals from what we know. He's never implied to be that much stronger than them.
Neither of those was a matchup issue. King and Queen were already defeated hence Greenbull was able to easily subdue them. That cannot be compared to Jozu who was full health and will facing Aokiji.
Greenbull vs Scabbards is again similar to the above, these characters were all injured with no haki usage. Yamato was even told not to fight. So again not comparable. You are heavily ignoring context to hype up Greenbull.
Law's feat suggest that. That one attack still has to land cleanly which is where the difficulty comes from.
Akainu is a level above Greenbull, for Fujitora, it is unknown since he is featless against a top tier. For Kizaru, he falls on the same level as Akainu and Aokiji based on portrayal and feats.
Oda has clearly separated the OJ admirals from the new ones(at least in the case of Greenbull).