Lol I've had issues like this before.
Funny thing is, I can't even find a legit tournament video where a Marth beats a link
https://www.youtube.com/watch?v=5JDdwDdratI
That's a really good one since Fly talks about the match-up during the video. You should all take a look at it
There's a local link player who's around my level so I have some experience in the match-up.
Shield angling is really important here. A lot of Link's moves poke Marth's shield, especially the feet (and some aerials hit Marth's head).
If Link ever uses up-b, remember it's NOT a multihit, therefore you can grab it after it hits your shield, even if Link is still spinning.
Shield in general is pretty good (if angled correctly). Link is pretty laggy so you can get a grab or wavedash after he hits you shield and you are not in much risk of a grab. Good Links will try to condition you and start throwing grabs after a while, so you need to pay a lot of attention to what's happening.
CC is really good against link too for the same reasons shielding is. You can CC boomerang and a lot of attacks to really high % (you can also hit his projectiles with fair or jab pretty easily).
Throw follow ups can be kinda awkward because of Link's heavy weight. D throw forces Link to tech most of the time so practice your tech chases. Up throw can force Link to jump which you can use to up air juggle him. If he doesn't jump, you can get a free up tilt into juicy follow ups. Mixing Link up with grabs is the best option IMO.
Another thing I've noticed most Link mains love to do is spot dodge. I have no idea why but they do (just look at the video for an example). Pretty free to punish if you expect it.
Edge guarding Link is kinda weird and sometimes there's not much you can do except reading him and hitting him offstage. Learning how to edge guard him will come with experience in the match up. Try to take him to stages without a wall so he can't use the hookshot.
Finally, here's another match
https://www.youtube.com/watch?v=3OQWGnVNe2c