Instructions
Brawls
Enter "Brawl" mode and play 400 matches (the Wii will save your progress over time. There's no need to play 400 matches at once). After the 400th brawl, Toon Link will challenge you to fight. Beat him, and acquire him as a character.
Complete the entire story mode (Subspace Emissary mode). Go back to the Forest level and look for a door in the beginning of the level that wasn't there prior to your completion of Subspace Emissary. Enter the door and fight Toon Link in a brawl. Beat him to gain him as a character.
Complete Subspace Emissary mode, then complete Classic mode. At the completion of Classic mode, Toon Link will challenge you to a brawl on The Great Sea stage. Beat him in the brawl, and he will join your cast of characters.