Ok, match 2 on battlefield
You're getting hit by so many things you shouldn't be. You're really just running around swinging your sword and hoping Sonic runs into it.
:14, you tried to hit Sonic on the platform with two up airs. Woah woah, slow down. You have the positioning advantage and Sonic is in a very tough spot. Try and up tilt or two, Marth controls the platforms and you are making it pretty easy for him to get down. When you get your opponent up there you want to keep him there for as long as possible. He gets down with relative ease and punishes your second up air.
:16, ok, he jab combod you and now you are now up against the edge. You have a sword. This is a good time to set up your zoning because you outrange him by a good amount. You hit his shield with a fair, ok that's fine. Personally i would have walked to dtilt. But then you randomly shield and allow him to roll away. What you should do now is walk towards him and try to reset your spacing and zoning, but instead you just jump and fair the ground. Sonic punishes.
:22, you nair'd the platform while sonic walks towards you and hits with an ftilt.
:24, you bair nothing and get hit
I'm not going to finish this, i just summed up your basic play style in just 15 seconds. Sometimes if you land a fair you double jump and try another fair. That's not a good idea. Fastfall once you land it and regain control of Marth. Airborne = less control.
Too many times your sword was behind you. You want to keep it in front of you and basically try not to get hit by abusing your disjointed hitboxes.
Marth can be used fast. But you have to be way smarter than this. Tilts are great.