For the most part, I get lots of trouble from fox and marth. I can sometimes get them off the edge but i need like something cheap because it's not that easy to rack up 130% damage and get a dsmash, by then I've probably lost a stock and some percent.
The best way to rack up damage is projectile combos. The bomb is your best bet for unleashing a powerful combo launcher because of the long hit stun it leaves etched on an opponent. Bomb plants work great on Fox at low % and for mid %'s throw a bomb into an Uair and then combo from there with another Uair or alternate into Utilts and Usmashes. When racking up damage against Marth, you're just going to rely on generic bomb and boomerang spacing into Nairs, dash attacks, and Dsmashes. They'll do just fine vs Marthy.
To get a kill move at high%, you'll either have to throw a bomb and run in for a cc Dsmash or run in for a dash attack. Or you can space Fairs and Nairs at their sheild and hope wait for a mistake out of your opponent to release a charged up Dsmash from your arial.
From here, You'll need to edgeguard:
For Marth, if he's recovering far and parallel to the edge/center of the stage, throw bombs, boomerangs and arrows to curve down slightly at his slope-shaped recovery with the sideB and disrupt his decent to the stage. If you are succesful with a hit from a bomb or an arrow, jump down and Nair him, because of the large hit stun lag of the bomb and arrow, and upB back to the stage. If your boomerang gets a hit on Marth instead then slide into a reliable edgehog or do the the lightsheild edgehog trick: it's brilliant vs Marth.
When Marth is already recovering low, same edgeguard strategy applies. Throw projectiles down at him and then either fall off the edge and Nair or you could face backwards (so young Link's face is in the direction of the stage) and WD back into a falling Nair. The other only other option than these would be edgehogs/lightsheild edgehog trick which work as well.
If Marth recovers far but very high up, then you should stay on the stage and jump high up and throw an angled boomerang to land a lucky hit or jump up over the stage and throw a bomb upwards. There isn't really that much else you can do but wait for him to descend lower and attempt to pounce on him with a Nair if he's still above you. Of course, if the Marth is any good he will simply outspace your Nair with Fair, but you should still try a Uair on him.
Marth has a step-by-step recovery in which he has two components of mobility that need to be combined just to get him to the stage: his sideB's (for horizontal movement) and the upB (vertical movement). Edgeguarding Marth comes through experience. When you knock one step or component out then he can't use the other to get back to the stage. Which means you if you can Nair his sideB's, then he won't have enough distance to upB back to the edge. Learn to play smart and abusive over his recovery, because remember, if you knock out one of Marth's steps, he falls down.
Now Fox:
Now as amazing as Fox is, he luckily doesn't have that amazing of a recovery in some terms. It's very straight forward and at times can be predictable or easily punishable. A good Fox will always try to land a sweetspot sideB phantasm whenever he can. If Fox does not land a perfect sweetspot with his sideB, then you should land a sweet Dsmash or jab right into his face. But, if the Fox player happens to be very good at landing perfect, sweetspot sideBs then either run off the stage and Nair, pull a bomb prior to this situation and Z drop it in front of the edge, or try and edgehog. It's very difficult to actually guard Fox when he succesfully lands a sweetspot sideB; sometimes you shouldn't even try to guard him at all when the situation seems to risky or pointless to actually get to the edge in time to guard him.
Fox's other recovery is his upB which is only used if he's too far underneath the center area of the stage or is simply too far for a sideB; only used in desperate situations, but is still commonly used. This is the type of recovery that young Link can handle very well. If Fox is directly underneath the edge but is very far down, then pull out a bomb, full hop over the edge and throw the bomb down at Fox, then fall back to the stage upright ready to unleash a Dsmash. You see, the bomb is used to stop Fox going for a perfect sweetspot and then the bomb can sometimes set the Fox up for a nice Dsmash. If Fox Techs the bomb then he'll have two options at that point: he can either fall down and attempt to upB back to the stage or after he Techs he immediately does a sideB right into the sweetspot edge. If Fox falls down for another upB, then pull out another bomb throw it down and wait to unleash a Dsmash, but if Fox keeps Teching the bombs and Dsmashes then let him keep Teching and keep racking up damage on him until he misses a Tech and dies (or if you screw up and let him back to the stage), then the process is an endless wash, rinse, repeat formula till one of you fails. Now, if Fox decides to sideB immediately after the Tech, then just Dsmash him. His sideB has start up time lag, so if you're quick enough you're Dsmash will hit him before he actually grabs the stage because he's floating there for a few frames.
This edgeguarding strategy is something I've invented (or at least I've never seen another young Link player use this edgeguard before) and it's worked extremely well against every Fox I've encountered. The only other thing, is that they never get the first Tech off and end up dieing my first Dsmash usually.
Another edgeguard is that you can hang off on the stage, wait for Fox upB upwards, and then ledge hop Nair or Dair him. Works moderately, but it isn't as effective as my edgeguard in my opinion.
Now there is an edgeguard against the upB with Fox is if he's very low beneath the stage but is NOT directly underneath the edge, but somewhere near that area where he would have to travel in a right diagonal trajectory to grab the stage. If Fox attempts to upB in this relative area, then run off the edge into a Nair or short hop into a fast fall Nair and hit Fox for a kill. Sometimes you'll be able to obtain a satisfying gimp kill.
I hope that helped!