Marth is another common character you will run into when playing competitive SSBM. He is very fast and has great moves that allow him to juggle his opponent when he catches their doublejump and boasts killpower when his smash attacks hit you with the tip of the sword.

Marth is going to dashdance a lot. As Link you'll want to throw boomerangs and bombs at him to force a reaction out of him and punish his reaction (or if they hit, then convert off of those and hit him). If he gets a grab and goes for Fthrow always DI away. If Marth does low Fairs on your shield he can safely dashback to avoid your shieldgrab. Similarly if he Dtilts your shield he can safely dashback to avoid your shieldgrab (you can shieldgrab if he opts for anything else after Dtilt, however). Generally speaking you'll want to be grounded and you'll want Marth to be above you. Do not go above Marth as he has some of the best anti-airs in the game and will juggle you if you burn your doublejump too early. In the words of Hunybear: “Link on the ground and Marth on the ground, Marth wins. Link in the air and Marth on the ground, Marth wins. Link in the air and Marth in the air, Marth wins. Link on the ground and Marth in the air, Link wins.”

Here's your moves that'll cause knockdown:

U-tilt - 16%
D-smash - 25%
U-air - 19%
Dash attack - 38%

Your flowchart should look something like this:

 
From 0-90% D-throw U-tilt
If Grabbed at 13% D-throw U-tilt will force knock down and tech on side plats
From 0-20% if Marth DIs behind you have to turn around U-tilt otherwise he can tech.
Past 20% if Marth DIs behind do not turn around.
From 13-30% D-throw U-tilt -> dash attack. ASDI down 
After 30% there are no guaranteed follow ups after U-tilt on DI away
Delay U-tilt after D-throw as much as possible
If Marth DIs D-throw U-tilt away from 13%-40% it will force tech on side platforms.
If ever Marth DIs in U-tilt then U-tilt again or U-air.

D-throw U-tilt after 50% just catch landings.

Bomb -> Dair...... 90s-120s Kill~90s
D-Throw -> D-air....... 80s-120s Kill~90s

Throw bombs and rangs if needed and run off and Nair him. Be sure not to get clipped by his Up B when Nairing him. Alternatively grab ledge and force him back onstage and ledgehop Dair his landing. Punish Side-B with nair or rang