I agree it wouldn't be impossible at all to do it in a day but it would be EXTREMELY tiring. Broken down what you need to do is:

• Complete 11 rounds of classic (Dr.mario Jiggs Y.Link Roy)
• Complete adventure once (Luigi)
• lose/win in VS or the 1P Modes with all the 14 characters unlocked from the beginning (Marth)
• Beat 100 multi-man melee (Falco)
• Clear 28 event matches (Ganon Pichu)
• Beat all target tests besides Mr. Game and watch and mewtwo
• Wait 5 hours for Mewtwo
• Beat Mewtwo's target test to fight Mr.GW Doing everything but the last 2 points should take around 3 hours tops, shame mewtwo drags this run down with a 5 hour wait fest.