fastest way - royal rumble mode. get a jobber deck with the one card you want to train. it will dominate as soon as you get it out there, could be up to 15 matches, the absolute quickest matches in the game.

Downsides: you may not pull your card until late, and theres no Special Move used, so you need to train that 20 some other way.

If youve got more than one card (like me right now, ive got like 3 pairs to train), Ring Domination is good. Youll get a bunch of matchups, sometimes your card vs. your card in a double stat bout. Specials are active. matches are quick. Just load up your 2-6 cards you want to train, and stick the one you most want to fight in the middle.

Other than that, youre just rapid-firing Wild mode.