I'm playing against a good Marth and he's good with that dancing move. He's never dumb enough to finish the attack when I'm shielding, so it feels almost nonpunishable. Am I wrong here, can I punish him even if he doesn't finish the move? Is it possible to start shielding after being hit by the first slash? Or can our A combo interrupt it in the middle?
First of all, Dancing blades (the move you are talking about) is not safe on shield. Which means you can punish it if he doesn't space correctly and you shield it, so yes, you are wrong. The key to punishing is anticipation. If you're blocking it, and you realize he doesn't try and finish it then you just know that you have to shield grab it faster, and you don't have to wait for the last hit. In fact, once the third hit comes out, if you shield grab quickly enough, the fourth hit cannot hit you before he gets grabbed. *This is only the case where he is in range to get grabbed, but since Marth steps into you, this will almost always be the case*
Atm you just aren't reacting fast enough. This isn't a knock against your natural reaction time (unless you do just have slow reactions), but it's just a sign that you need to look for/concentrate on certain visual cues. For example, knowing what I told you about the 4th hit being punishable, you concentrate on waiting for the third hit to come out. Or using your previous experience against him, you wait for whichever hit he usually stops at to come out, and then you can immediately input your punish. This is why I say anticipation is key. You can react much faster if you expect something to happen by focusing on other visual cues, then just playing randomly and not thinking/anticipating at all.
Nothing can interrupt his combo once he starts it unless you SDI like crazy or he inputs the slashes very slowly. But on shield, it is definitely punishable.
LOLOL