im feelin a little too lazy to type my marth advice, but i'll give you cliffnotes.
1) CC only works if they space poorly. M2k let one of the biggest cats out of the bag about doc vs marth. As long as marth properly spaces aerials (more specifically n.air) doc cant do ANYTHING vs the aerials. You cant CC d.smash cause you're out of range.
2) Speaking of aerials; as long as he's l cancelling them, Dont cant really do alot to punish them in general.
3) Contrary to what people think, you initially can make marth approach you because of pills. However, once you get just outside of his tip range, you have to approach him. (for those who like to go with the initiave-response theory of fighters.)
4) Unless Marth will let you, you have to pill to approach him, and then figure out what he's going to do after he deals with pill. As long as he has room, his best response to pills will be to move (because he stays in neutral, never entering ANY recover frames).
5) In spite of what some people try to say, you can combo marth. You can u.air juggle him. But whatever God you pray to wont save you if you try to cheat marth and start early.
6) You can d.throw -> f.air him. 90-110% or so is SH, 110%-135% or so is FH. Something like that, i dont remember numbers, i just know "ARound 100%, d.throw -> f.air -> profit."
7) And the absolute most imporant cliffnote i can give you about marth iiiiiiiiiis.
If marth ever. EVER. ever. F.smashes and f.tilts and you block, you are allowed to wavedash in. If it's tipped, i think you're guarenteed a wavetilt (im not exactly sure, i never think about this situation and i'll have to try it). However, if it's untipped, you are guarenteed a wave(down)smash.
EVERYTIME MARTH F.SMASHES AND YOU BLOCK IT, YOU WAVESMASH
EVERYTIME MARTH F.SMASHES AND YOU BLOCK IT, YOU WAVESMASH
EVERYTIME MARTH F.SMASHES AND YOU BLOCK IT, YOU WAVESMASH
EVERYTIME MARTH F.SMASHES AND YOU BLOCK IT, YOU WAVESMASH
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That's it for my marth cliffnotes. I'll type more about it later if needed... and when I stop being lazy.