Easiest method is to just go into a local game, change the game settings to 1 stock mode, and just walk off the stage when the game starts. It doesn't matter if you win or lose so this way a battle only takes around 2-5 seconds (depending how fast you can run off). Once the match is over you can just select the option to play again and keep doing this until you hit 50 games.
If you select a random character as your own you also have a very high chance of completing the Say My Name! achievement simultaneously!
And if you do this 50 more times, you'll unlock the Defender of the Earth achievement as well!