An easy way to unlock every character [if you don't like adventure mode], and other achievments.
First, go to Special Brawl. Set it to "Stamina", "Flower", and "Fast". [I think I got all those right]
Next, choose your character. Go against 1 CPU. Set the CPU's stamina down to 1. Now, start the match. Instantly, the CPU will die.
Using this method, you can unlock characters after a certain number of brawls. You can also face certain opponents to unlock acheivements, stages, etc. This will also add to your play time [more achievements]
To use the method faster, just keep pressing the START button [or +, etc.]