When I play this MU, I feel like it's in our favour but the Marths I play aren't exactly spectacular so I'll go with 0. Take everything I say on this with a grain of salt cause I guess you could call me inexperienced in this MU.
-We're better at juggling, better offstage and better at killing. Marth has the advantage when it comes to damage output (mainly because of dancing blade) so DB plays a very important part in this MU. If you can bait out DBs and avoid them, shooting paralyzers is a good way to punish because they won't clash with DB as DB is transcendent, and from there you can follow up with dash attack-->something (probably utilt)/grab/sideB (obviously a good Marth won't miss DBs often). I feel paralyzers are good in general as they can trap Marth's landings quite well and they hinder his approaches.
-Run away--> pivot nB/sideB are very good in this MU I feel, they help us space ourselves against Marth.
-Marth should never be approaching with rising aerials; if he does, utilt/usmash/uair.
-Landing fairs can be powershielded-->utilt/dtilt/dash attack (or at least it seems like it, I do it all the time lol) depending on how the Marth spaces it.
-Don't charge dsmash too often, Marth can often nair you for it.
-The entire jab combo works on Marth (ie he can't powershield the 3rd hit unless he SDI's down) but he can dolphin slash out, so...yeah...XD
-Don't approach with sh sideB.
-I think our bair outranges his fair.
-When I SDI out of poorly spaced dancing blades (I get hit by the first 3 hits, then the 4th misses) then punish with dtilt/dash attack. I don't know if you can punish with sideB/grab, I haven't tried.
-Spacing with dtilt is very important.
-Don't use your get-up attack after 100, Marth can upB OoS. This probably works for our regular get-up attack as well (I'm talking about the get-up attack from the ledge)
-Marth either has to commit to kill, kill you at high percents (150%+) with non-tippered aerials/DB or get a hard punish, so be safe on shield once you reach 80/90%. Less utilts, grabs and sideBs (excluding run away pivot sideBs, those are still good). It should be obvious that you want to avoid tippered aerials at all times.
-At low percents I THINK you can avoid fthrow followups (fthrow-->nair and fthrow-->fsmash) with downB, but don't get predictable with it. Pretty sure you can't get CGed.
In general, just avoid DB at all costs. Punish landings with dtilt, dash attack and paralyzer. Punish anything unsafe on shield/whiffed with dtilt/utilt/dash attack/uair(or dsmash, but this shouldn't be happening lol). Zone with run away pivot nB/sideB, and space your dtilts. Uair OoS is good. Juggle the crap out of him at every given opportunity.
We can't edgeguard him THAT well (we're probably not going to land any bairs/2nd hit fairs) so don't get too crazy. Most of the time you can't really get anything other than an uair, so just use that.
Ugh, okay I'm gonna sleep now.