To add what Cactuar's saying, the u-air maneuver is just a mindgame to force the opponent into the air. A good Marth is always one step ahead so never assume your attack is going to land; better yet, it's better to assume the worse so you're always in position where you have the advantage.
So if the up-air is shielded, the opponent will:
1.) Do nothing. In that case, an up-tilt will shieldstab them.
2.) Release their shield, fall through platform and attempt to hit you. Again, you up-tilt or pivot grab, but you can see them trying to do this a mile away. Pretty silly if you ask me, but that kind of **** works on me.
3.) Jump out of shield to a safer position aka a higher platform. Ultimately, your opponent will be doing their best to get lower ground, but the 2nd best option against Marth is the highest ground where they can't be up-tilt combo'ed.
Idk, Marth's air game is all about tricking the person into jumping or using their double jump, then begining the aerial rapsies to edgeguard, f-smash, or both.
Marth's ground game is all about tricking the opponent into getting grabbed. That's about it.
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