How do you get mr game watches arena in Super Smash Bros.?

Mr. Game & Watch should be the last character you unlock in Melee (unless you unlock the secret characters out of order).

To unlock Mr. Game & Watch:

• Play 1,000 vs. matches (that's whole lot of playing)
• The easiest way is to play all the characters (including the secret characters you already have) in Classic Mode or Adventure Mode.

Once you beat Mr. Game & Watch after you do either one, you unlock All-Star Mode, where you must beat all regular and secret characters with only 3 hearts to win.