I play Mario and one thing that really pisses me off pretty bad is a random WD dsmash they're very annoying to Mario. Also when if that Mario is spamming his cape he = noob lol. Another thing my friend uses against me is that slap in the air because the only really attacks you can counter that with is bair and uair and the bair still ties with the slap so you get more damage off. also off the spawn you can tornado at 0 damage and juggle for just a little bit (tornado - utilt - slap = first 20 damage). Spacing is also very well against Mario because hes kinda fat and cant move well and Mario has bad range but yet again so does Luigi unless you WD the majority of your attacks. You should also be able to tornado recover because if your coming back with your the torpedo it makes Luigi VERY easy to spike i get my friend 6/10 times like that at least once a game so save your jump and make sure you can tornado recover. Also its better to take Mario to flat maps with no platforms because Luigi can rip Mario apart if he doesn't have platforms, platforms for Mario make it very easy to win against Luigi IMO so pick maps like FD, Mute city, Dreamland (sorta), DK64.
I think i covered everything.
Edit: also use a lot of nair against most combos. It is sooo very annoying but it works very very well against Mario's combos