The only way to prove you’re the best is to face off against another opponent alone – it’s you versus them, and nothing between you. Call of Duty Mobile had a 1v1 mode called Duel, and when it was first released, it featured a map called Saloon. It was a simple matter of joining the game mode and facing off against another player. The two of you would fight it out to see who was the best. That match mode was not available forever. Now, it appears the only way to battle against another solo player is through a private match.
Proceed to the multiplayer lobby and join or invite another friend online. When you know the two of you are ready, click the three-dashed menu icon on the top right of your multiplayer screen to bring up the dropdown menu, and then click “Private.” It will bring you to a private match screen. All you have to do is invite your friend to the game, and the two of you are good to go. Before you start the fight, you can set the match settings to adjust the kill and time limits. You can even have others spectate the match.
When you’re ready, hit the start button, and the two of you are good to go. Until a proper 1v1 match mode becomes available, this is likely the best solution. It’s probably impractical to use this method to fight other random players. However, you can invite players you have recently played with in other matches. If they accept, have at it.