You can restrict the level of your opponent when searching for an arena game.
This will ONLY work if you queue from the "Play Now" page, NOT the page of the game itself.

Go to the "Play Now" page. Make sure "Arena" is selected. Click the cog icon for the game you're interested in to open up your queue settings.

Restrict your opponent level based on the ARENA LEAGUE that you are in.
You are in Diamond League.
If you only have "My level" in green then you will ONLY be matched against people in Diamond League.
If you have "My level" and "My level +/-1" then you will ONLY be matched against people in Elite League, Diamond League, Platinum League.
You can NEVER restrict arena opponents by Elo, that's not how arena works.
These settings will not be remembered if you try to queue from the page of the game itself. Only queue from the Play Now page.
This will ONLY work if you queue from the "Play Now" page, NOT the page of the game itself.

Go to the "Play Now" page. Make sure "Arena" is selected. Click the cog icon for the game you're interested in to open up your queue settings.

Restrict your opponent level based on the ARENA LEAGUE that you are in.
You are in Diamond League.
If you only have "My level" in green then you will ONLY be matched against people in Diamond League.
If you have "My level" and "My level +/-1" then you will ONLY be matched against people in Elite League, Diamond League, Platinum League.
You can NEVER restrict arena opponents by Elo, that's not how arena works.
These settings will not be remembered if you try to queue from the page of the game itself. Only queue from the Play Now page.