The Ice Climbers match up is literally about not getting grabbed. You have to run for days and just get chip damage on them until you can separate them and kill Nana. Then, you either choose to kill Popo or time him out.
The ledge is your friend against the ICs. ECE messes them up pretty well because of Nana's shield delay, and we have good options for getting onstage past them as well (at least, not getting grabbed/killed when attempting to return, which is fine by me). Ledge drop > DJ whatever and Ledge Hop > DJAD are extremely good at getting past their ledge traps.
Otherwise, abuse safe options against them. Mix up pivot grabs with PURE Bair rushes so you don't land anywhere near their shield. Egg Lay is a godsend when landing. Against SoPo, we have GR > Usmash for the kill.
For Marth, the MU at neutral all comes down to dealing with his spacing options. Basically, we cannot DIRECTLY beat out Fair or Nair or whatever. We have a few options, however. We can throw eggs to cover the aerial option, although he can read this and simply dash towards us and dash attack/Dancing Blade. We can also preempt the aerial in two ways: running Usmash or Bair rush. Both need to be timed so that Marth doesn't have enough time to prepare himself to retreat and swing Fair to stuff us out. There's an interesting spacing to it, but it's difficult to describe, so you'll need to play around with it. Usmash can also be used to duck under a Fair swing and punish, on occasion. Our third approach option is simply dash grabbing him as he lands. I recommend throwing him towards the ledge so we can ledge trap him.
Defensively, Pivot Grab isn't as powerful as normal unless the Marth commits SUPER hard. The arc of Fair hits our blindspot reliably, so you usually need to overspace your grab by running back further than usual. Dancing Blade stall also helps him avoid this.
If Marth gets momentum on you, retreat as far as possible. The ledge is okay in this MU. ECE are a decent tool for racking damage, just be careful to avoid getting Fsmashed on the ledge for it. Getting off the ledge can be kinda tricky, but our standard options are decently effective. Marth ledge traps pretty well though, so expect some opposition.
Alternatively, when we get momentum on Marth, we need to try to carry it as far as possible. If Marth's back is to us, you pretty much can do what you want. If he's facing you, it's best not to jump right in because he can just Fair to swat you away. Try to force him to the ledge and then start frame trapping him with eggs. Lightly tossed eggs cover all his ledge options extremely well, and if he airdodges through you can just punish it.
The main trouble with the Marth match up comes in killing. Neither character has safe killing options on the other. This means that if you're a stock behind, you'll either get a revenge kill quickly, or eat a lot of damage trying to kill him to even up the stocks. DJ Armor > Nair is a good revenge killer if you need it. Either way, whoever takes the first stock in this MU usually ends up with a pretty large advantage for the rest of the match.
Also, last stock last hit against Marth SUCKS.
