I'm gonna comment on Marth v. Roy since I play both characters..
Stay grounded. Repeat. Stay grounded. His side B is actually quite wonderful at stuffing your aerial approaches, and he can get some decent damage after it. Short hop walls have substantial risk because his dashdance is comparable to your own. In this way it is similar to the Marth ditto. Platform camping may seem tempting because he can't really hit you when you are up there, but its very tough to counterattack either; side B and dashdance are that big of threats.
As Dart said, uthrow. Roy is so so so bad in the air. Combo him like Falco. Edgeguarding is rather simple but if your having trouble remember runoff counter. Crouch cancel his upB; don't lose positioning if you can't get the edgeguard.
If you are in a tricky spot, just shield. Take the grab, its not that big of a deal. You should live forever in this matchup as long as you don't get hit by a random fsmash. Whenever Roy sees someone in his shield all hes trying to do is bait something to get a gimmicky fsmash. Don't fall for it and be patient. Lightshield and angle so you don't get shieldstabbed. Eventually he'll dance around your shield so much that you can just do something OOS and **** him.
If hes in a bad spot, just crouch outside his shield. Looks for WD OOS, roll, and (possibly) upB OOS. All of these are super easy to cover and he'll have to choose one out of fear of getting shield stabbed by dtilt. Roys tend to be very defensive after a tech so don't worry about a hitbox being thrown out. He WILL shield/roll/dash. Remember that Roy hates to be on the edge so on techchases make covering roll into the stage a priority. Cover tech in place at the edge with a dtilt; thats so often a stock that you don't need to try reaction grabs/fsmashes. Even if you somehow miss you'll just get his shield and hes still effed. Poor Roy.
Like most low tier MUs, dtilt is your best move. WD dtilt, dash dtilt, walk dtilt...do it all. It may look like Roy has similar range as you but he does not. Push him to the edge. Don't gamble by going in with grabs as much as you're used to; Roy is actually rather hard to grab. Just be patient and push push push. The stock will evaporate once you get your opportunity. Dashdancing is awesome as always but tbh I'd be careful with it. You want to keep dtilt as a threat. If you really want to approach, try dash attack. Its pretty safe against Roy.