1. Beat Classic Mode on Easy without using any continues
2. Meet Luigi in the Subspace Emissary
3. Complete 22 Vs. Matches
If you are reasonably good at Classic Mode, it isn't too hard to clear it without using any continues. This may be the fastest way to earn him if you have the skill for it. You will also eventually meet him as you play through the Subspace Emissary. The easiest way is probably to just play through 22 quick Vs. battles set to 1 stock. Once you challenge him, he is usually a pushover. Give him a good beating to unlock him.
Every man dies, but few truly live. Live your life to its fullest, every day as if it were your last.