I will only post the most useful way, so get ready.
Luigi: Complete Classic mode with no continues.
Ganondorf: Complete Classic mode on Hard difficulty with Link or Zelda.
Toon Link: Play 400 Vs. Matches(Special Brawl, Stamina set to 1(300 for you.) Flower on the head, just choose random stages. Takes less than an hour.)
Falco: Play 50 Vs. Matches.
Wolf: Play 450 Vs. Matches.
Captain Falcon: Complete Classic mode on Normal difficulty in under 12 minutes.
Lucario: Play 100 Vs. Matches.
Jigglypuff: Play 350 Vs. Matches.
Marth: Play 10 Vs. Matches.
Ness: Play 5 Vs. Matches.
R.O.B: Play 160 Vs. Matches.
Mr. Game&Watch: Play 250 Vs. Matches.
Snake: Brawl 15 times on Shadow Moses Island.
Sonic: Complete Classic Mode(Any difficulty.) with 10 characters.
I really hope it helped, 'cause this is how I unlocked all my characters!