Honkai Star Rail 2.1 Story RECAP 

It is pressumed that you have to break the cornerstones to get the full power of the cornerstone, Aven shattered it before coming to Penacony ("It is said that the Ten Stonehearts must cut their Cornerstone with their own will in order to allow its brilliance to radiate"). So that should be the full power of the Aventurine Cornerstone afai,k. And, the cornerstone is only a part of Diamond's power, while Acheron herself has the full power of an emanator (While the Ten Stonehears only have some/ parts of an emanators power). So, Acheron would prob win.