Here's a trick I used, go into smash, set rules to have 1 stock, then walk off the edge.  You should get a new challenger, defeat them, then close out the game and go back into it, repeat.  You should keep getting new challengers each time you fight, instead of having to wait the normal 10 mins.  If you lose, don't worry, they have a thing called Challenger's Approach with the other game modes, where you can rebattle challengers you lost to.  I've been having trouble with Metaknight, so I keep going back to it after that.