One thing I absolutely love doing against macs, is downthrow resets. They never see it coming the first few times, and it's hilarious seeing them flail around in the air.
No disrespect twords them though, I like fighting smart mac players. The thing I find about this match up is, you are just as vulnerable as mac is in the air. Luigi falls slow, and moves left to right even slower, not to mention any of his aerials can just get hard countered by super armor, very bad news. I don't recommend short hopping too much, this matchup really put a bruise in that one for me, completely devoured that habit that I've had.
Gotta keep your feet on the ground and look for grab/shield options, force rolls by manipulate him with confusing movement, if he doesn't like to roll, throw some fireballs out and look for his side b, and get him off the stage. Remember to time your punishes thoroughly, if you find mac whiffing an attack, chances are he'll be expecting you to dart in for a punish.