1. Get R.O.B. to join your party in the Subspace Emissary
2. Get 250 Different Trophies
3. Play 160 Vs. Matches
It seems that the easiest way to unlock most of the characters is to simply play your way through the Subspace Emissary mode. Once R.O.B. joins your party in this mode, he will be yours. If you are a trophy collector like myself, just keep adding to your collection until you hit the 250 mark. Of course as always you can unlock him just by playing a lot of Vs. battles. As far as actually defeating him, I can't offer much advice yet. I'll post more when I have him. I will post more when I have more information.
Every man dies, but few truly live. Live your life to its fullest, every day as if it were your last.