The start of The Two Plumbers and their Counterparts Arc. Luigi and Mario have worked side by side for a long time. They know how each other thinks, which is why they’d make dangerous opponents. They each know how to counter their opponent’s skills.
Mario is an all around fighter who has been fighting for years. He knows how to fight just about every kind of enemy. He has also attained many different super forms over the years. In terms of battle experience Luigi will never catch up to Mario.
Of course that is Luigi’s slight edge. Because he hasn’t fought as much as Mario, Mario won’t be able to predict his moves as well as Luigi will predict his. Luigi can also tap into Mr L power. Of course in the end for all of his power Luigi just won’t be able to defeat Mario. The Arc is young and Luigi will be back. Mario wins.
Fanfic version below