Start off by going into vs mode and setting it to Stamina Flower. Give the AI 1 HP and yourself more than 1 HP. Pick Donkey Kong and play on Pokemon Stadium 2 for 10 matches. After five matches, you'll unlock Ness. After ten matches, you'll unlock Pokemon Stadium (melee) and Marth. Switch the stage to Yoshi's Island (melee) and play 10 more matches. After three more, you'll unlock the Super Mario World demo, and after ten more you'll unlock Jungle Japes and 75m (DK arcade). 20 matches total.

Now switch your character to Kirby and play 15 matches on Shadow Moses Island. After two more, you'll unlock Luigi, and after the fifteen you'll unlock Snake and the Snake Eater music. Now switch to the Halberd and play five more matches to unlock Green Greens. 40 matches total.

Switch to Peach and play five more matches still on the Halberd. You'll unlock some music (5 matches) and the Super Mario Bros 2 demo (3 matches). Your character no longer matters, but switch the stage to Mushroomy Kingdom and play 10 more matches. After 5 matches, you unlock Falco, and after 10 matches, you unlock the underwater Mario music. 55 matches total.

Play the next 10 matches on those default stage builder stages to unlock stage parts A. Play the next five on Norfair, and then you'll unlock Captain Falcon. Play five more on Norfair, using Captain Falcon this time, to unlock the Ending (Metriod Music). Switch to Port Town, and after five matches you'll unlock Big Blue and the Peach's Castle (melee) music. Your character doesn't matter anymore, but play five more matches on Port Town to unlock the music track "Dream Chaser". 85 matches total.

Switch to Frigate Orpheon and play 10 matches to unlock the Metriod Prime 2: Multiplay music, and then switch to Castle Siege for 10 more matches. After 5, you'll unlock Lucario, and after 10 you'll unlock the track "Powerhungry Fool". 105 matches total.

Unfortunately, things slow down a bit here. Character and stage don't matter, just play a bunch more matches. After 55 more, you'll unlock R.O.B., and after 95 more you'll unlock Ganondorf. After unlocking Ganondorf, play 50 more to unlock Mr. Game and Watch and Flat Zone 2 and 100 more to unlock Sonic and Green Hill Zone. 300 matches total.

Play the next 10 matches on Green Hill Zone to unlock some Sonic music, and then continue playing matches on any stage until you unlock Jigglypuff (350 matches total) and Toon Link and the Great Sea stage (400 matches total). Play the next 10 as Toon Link to unlock the Ocarina of Time demo and then 40 more with anyone to unlock Wolf, All Star mode, and the Menu 2 music. 450 matches total, and you now have all characters.

Now let's get the remaining stages. Beat Classic on easy with anyone to unlock Luigi's Mansion (note: you can do this faster by using Luigi in 3 brawls) and the Ice Climbers (melee) music. Head over to the events and beat event 8 to unlock the "Road to Tokiwa" music and keep playing event matches until you unlock the next set. In that set, beat Event 19 to unlock the Mario Bros. stage, event 13 for some Kirby music, event 15 for yet more Kirby music, and event 18 on the difficult setting for some Zelda music. Play a few more events to unlock the third set. Beat events 25 and 28 to unlock Spear Pillar and the Electroplankton stages respectively. Congrats, you have all stages!