You must be out of your mind if you think Isshou can stand a chance against the likes of Akainu.
It's like comparing prime admiral Sengoku or Kong to Akainu, Aokiji, or even Kizaru.
Old admirals>>>>the new
This isn't true, the New Admirals (Akainu, Aokiji, & Kizaru) should inferior to Old Admiral (Kong & Sengoku), because they all from Marine. But, Isshou and Green Bull from outside Marine, so we cannot said they inferior to Old Admirals. Its will be similar if we said The 7th Shichibukai inferior to Old Shichi, because he is new.
Isshou & Green Bull = Other Admirals (until it proof otherwise)