Metal Blade?
Of course, then she'll start doing the gravitational pull.
You're kinda screwed in MM vs Rosa unless you try to beat her at the physical game. In that matchup you're at a significant disadvantage since only 6 of MM's attacking moves aren't projectiles.
The pull is definitely the first problem... My general strategy going in to most first fights (where I have no idea what's going to happen) is to use Crash Bomb and Metal Blade as a smoke-screen; if the Crash Bomb hits, I'll time out for the explosion or rush in for a grab or Top Spin if the opponent wants to shield-wait it out, or do another cross-up while running right at them if the Crash Bomb misses. For obvious reasons, Rosalina and Luma locks both of those out thanks to that gravity pull.
I wish I had saved the replays of the matches I was having as I was getting closer to beating that particular player by the time the third round came about. Flame Sabre remains the best friend, at least to lock down a ninety-degree angle for me to force Rosa to ether back-away (and where I can catch her with a Dash Slide) or roll behind (a Smash Down B can clear her up, and if I'm fast, the spark can catch her and we're playing aerobatics).
Definitely a frustrating match-up. Hoping to get more done with it as I do think it's an uphill battle but there might be something to hash out.