Firstly, use the stickied Question and Answer thread for this.
Secondly, the Marth you are playing is clearly a scrub. If he knocks you up into the air with dancing blade then airdodge past him when he tries to follow up and utilt/usmash him. If he tries to nair and fair you while you are on the ground, and you know he is going to, shield and press A after he hits (it'll grab him). Make it a point that whenever he gets too close and tries to hit you the same old way he always does, that you simply shield the attack and punish it with a grab.
Also if you see that he is going to break through your projectiles then get on the ground ASAP and hold your shield. He's clearly not doing the shield-poke version of Dancing Blade, so just shield his attack and grab him.
There's something called telegraphing, which means a player is planning out their moves in advance and will follow through on that attack sequence to the end, whatever the opponent ends up doing. Your opponent is telegraphing because he only ever does the same version of Dancing Blade whenever he gets in close. You know what he's going to do, so just plan a way to punish it (such as shielding).
Make sure you don't telegraph either. Zair is nice, but if you feel he's going to break through anyways, don't keep doing it. If you find yourself in a position where you know your projectiles won't hit him, don't use them.