My reply will consist in a couple of points:
1. Counter. There was a lot of times where sheik was just charging her fsmash, and you just stood there, sometimes I like to take it to a greater level and jump into it with a down b. Or when sheik is chasing you in the air with her fairs, you know that the person was using the same tactic thought out the whole fight, throw in a counter, and the sheik user will think twice.
2. Sheildbreaker. Although I didn't notice the sheik player shielding, the shield breaker has massive range, and if you tip it properly, it has awesome knock back. There were times when you could have just pressed it once and sheik would not be able to recover from it.
3. Double Fair. The way I play Marth is that when I have a chance to do a double fair, I go for it. A lot of the times the opponent rolls away from it causing the 1st fair to miss, and then shortly after if you do your second fair while still keeping the momentum and moving forward, you will land the 2nd fair from which you can do your ftilt or your fsmash, maybe even the side b combo.
4. Spike. Although you do play defensively, there were times when you could have spiked sheik right before she grabs the edge, and you wouldn't even have had to leave the ground really. Just SH and do a dair, the sword will go though the map and should hit them. Its very situational, but I've done it a thousand times on my friend's DDD.
Thats it, feel free to take what you want from this post, also post any concerns about my "theories". =D
*Edit*
After looking again, you should also grab. You can do a forward grab, then follow up with a double fair and w.e. you want after it.