Marth vs Fox
Common tactics used by Marth and counterplay
Dash back grab
Explanation: If Marth expects Fox to directly attack him, he will often use his strong dash back to avoid the attack then whiff punish with grab. Marth's dash back is especially strong because the animation shifts his character model drastically in the direction of the dash immediately.
Counterplay: If Fox expects a dash back, place an attack behind where the Marth currently is to where he would be after a dash back (AKA overshoot). Any attack can be used overshot to punish dashback, but using overshoot shine is especially strong for a few reasons: it works at every percent, leads to grab or up smash, and since it is active frame 1 you can decide last minute not to attack. If you feel like you are overshooting and are still getting dash back grabbed, you aren't aiming far enough behind Marth.
Late fair
Explanation: Marth's will often use late fair to make it difficult for Fox to approach. The reason that late fair is so effective at preventing Fox from approaching is that there are so many different timings and spacings that Marth can use to make it difficult to correctly whiff punish. Fox has to consider if Marth is fast falling or not before the fair. If Marth doesn't fast fall and Fox attempts to whiff punish fair at the timing of a fast-fall late fair, then Fox will be too early and will get hit by the no-fast-fall fair. If Fox whiff punishes at a timing that would work for no-fast-fall fair but Marth fast falls, then Fox will be too late and will likely get dash back grabbed. Marth can also drift forward or backward in the air before the fair, which also changes the timing/spacing for a successful whiff punish. Drifting forward can cause Fox to get hit by the fair, even if he guessed the correct fast-fall or no-fast-fast timing. When Marth combines fast-fall, no-fast-fall, drift forward, drift backward, and no drift it becomes very difficult for Fox to get the right timing and spacing to whiff punish late fair.
Counterplay: