Given Luigi's character, I doubt he'll actually fight back.
Boxing stat wise ... Luigi has a huge reach advantage, age is about the same, height to Luigi, weight maybe goes to Mario? I have a feeling Luigi is faster on his feet too.
He'd just jump over him. Faster, a fan favourite, and so long as mario can't wall jump over him, no way for Mario to ground pound him. Logically, not going to go Mario's way. Instead of this, maybe get Super Smash Bros- although come to think of it, neither seems to win more often.
Faster, can jump higher, all-around better in a fight. Even if he's a little scared.