Select play offline from the main menu.
Game mode: Arena
Switch mode: Linear
Play against one bot. If there is more than one bot in the lobby, press start and put ‘bots slot fill’ on 2.
Start the game. You can go afk, the bot will kill you or itself usually rather quickly. Just make sure you start another game when your current game ends. That way you can find something else to occupy your time while you’re waiting. You can expect this to take around an hour for 50 matches.
To get this achievement as fast as possible, you can pick up the controller and immediately kill yourself every round, but it really doesn’t save that much time and it gets tedious.