How to get an Everstone from the Mauville Inverse Battle stop?

So I’m trying to get an Everstone from the Inverse Battle Stop but I can’t seem to do it. All I have gotten are Oran Berries (when I did bad but won), a Colbur Berry (I don’t remember how), and Sitrus Berries, which I seem to get for doing really good. I just can’t seem to get an Everstone. All that I can find on Bulbapedia is this:

After he is defeated, Inver will give the player an item depending on how well they did.

However, that doesn’t help me much. Can someone help me with this?

Pleaselog inorregisterto add a comment.

Inver’s evaluation of the player is based on a score calculated by subtracting the number of not very effective hits the player made from the number of super effective hits they made.

You need to score between a seven and a nine to get an Everstone, but there’s also a chance to get all of the other evolutionary stones. Assuming the chance of getting each of them is equal, you’ve only got a 10% chance of getting one, assuming you score between seven and nine. That combined with the fact that you can only battle him once a day, you’d be much better offusing a different method to get an Everstone.

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