First, don't come into this MU feeling like you have to approach all of the time. This is exactly what they want and you'll just get poked to death. Prioritize keeping center stage and throw fireballs at them to force approaches. Play passively, defensively and campy. Now, I don't want you to think that fireball automatically wins the MU for you (it doesn't) but it can put more pressure on Marcina to approach which is what you want, especially if you have center stage. Fludd can be good for pushing them back and getting center stage as well. Reading how they react to fireball is important too. If they stay in shield, approaching with FH fireball is a good option. If they jump you can either back air them (if they use mostly landing aerials), run underneath them and Usmash, or run close to them and shield to throw off their spacing.
Second, respecting their spacing and whiff punishing their attacks is great advice. Whiff-punishing is easier if you play a character with better mobility than Marcina, like Greninja for instance, but Mario can do this as well. You really have to pick your spots, read your opponent well, and be evasive.
Finally, make sure that your punishes count. If you get a grab or landing aerial, make sure that you get as much as you can out of it. Mario's main advantage in this MU is that he does way more damage to Marcina than vice versa.
