I totally think against a good link, the marth/link matchup isn't as sloped as it's thought to be.
After a well spaced fair or dtilt, into their shield I mean, you can jab and it'll keep you from getting shield grabbed.
If you think they're going to grab/shield grab, jump! Link's grapple can't grab you if you're in the air.
If you're approaching and they have a bomb, a.k.a., they're going to throw it, cancel your dash into a crouch and dtilt. goes under the bomb everytime.
I just had a thought, you could fsmash 'em after the bomb whizzes over your head, but I'm not sure, they'd prolly be able to put their shield back up.
When edguarding link, if they're the kind that likes to sweetspot grapple the edge, hang onto the edge and ledgehop bair when you see him start with the airdodge
If they're up-b ing, you can either ledgehop dair, or if you're on the stage, make sure you're back is facing towards the edge while you're waiting for the right time to fsmash. then fsmash towards him
I figured that out by experience, it really sucks for link if you fsmash him with your back towards him before hand. he can just keep ledge-teching and you're out of reach.
if you're at a really high percentage and you're d-throwed, di behind him. less kill options for him.
when thrown off stage/recovering, don't use your side-b unless you absolutely need to. (more general tip, but link has it easy by just running off the stage and jumping off with that ridiculously long-lasting sex kick of his and gimping marth. you're safer though if he's hanging on the ledge, trying to make you up-b onto the stage so he can ledgehop dair you. counter that by forcing him off the edge by threatening to hit him with your side-b. it's like saying, "I'm just floating here with my floaty side-b attack and if you don't get off soon, you're invincibility's going to wear off")
I know my link matchup, trust me. haha
also, it helps if you can practice grabbing thrown bombs