A few do's and don'ts that I try to follow...
1) DON'T stay on a platform above marth. You will get U-smashed, and die.
2) DON'T stay in the air above marth. Move or QAC back to ground, unless you like being U-aired.
3) DO spotdodge F-smash instead of shielding it. When you shield, you're pushed too far away to punish.
4) That said, DON'T spotdodge the dancing blade. It won't end well.
5) DO edgeguarding with just T-jolt and thunder. If you go out for the N-air, you'll eat an F-air or a Dolphin slash.
6) DO keep marth above you. His D-air is a weakness in his game. So, when you do grab him, and you're out of combos (D-throw to N-air and F-throw to U-smash don't work any longer), U-throw or B-throw might work.
7) DO remember that U-move to thunder is always awesome, if you can pull it off.
8) If he whiffs with an Dolphin slash on-stage, DO punish quickly. I personally like to jump and N-air.
9) Basically, if he's up really close, DO shield. Usually he'll Dancing blade or Dolphin slash.
A quick question...
Given how ridiculously large Pika's QA hurtbox is, is it possible for Marth to guard the ledge? Once I took a fully charged shieldbreaker while QA'ing to the ledge. It wasn't fun. I want to know if that was user error on my part, or a tactic the Marth's could use...