You can turn on fight requests when setting up the difficulty of your arcade play, as well as the time at the bottom of the screen. Make sure it is set to yes, and simply play through arcade waiting for the prompt that new challengers have appeared. These also count as ranked matches, so anybody searching that meets your criteria will interrupt your session and you will fight them. If you can, have a friend search for a ranked match as the multiplayer for this game is practically dead these days.