Vs Razor Leaf, use running shield. If you shield during a dash, it will stop your momentum, but if you shield during a run, it will keep it, letting you slide through the first few hits of the razor leaf and punishing Ivysaur.
You can also clank it if you were too slow to react, but that just lets the Ivy player approach or shoot another one.
If you have the center of stage, dash dance camp her and wait for opportunities. Basic Marth stuff here I don't think I have to elaborate.
In that matchup, you can and should jump a lot. Ivysaur's oos options suck harder than anything you've ever seen since Smash 64, so that makes tipper [insert any aerial] safe on block, and puts you out of Ivy's dtilt's way.
Yes, I know this is not how you play Marth in Melee. But this is how you play Marth against Ivysaur.
Ivy's bair sucks in the neutral. It doesn't combo into anything, and can be shield grabbed and crouch cancelled. The only valid reason to use it in the neutral is if you're close to the ledge and/or at a high enough % that it will set you up for an edgeguard. But seriously, don't fear the bair. Your fair is basically Ivy's bair, except four times faster and with followups. Profit.
Comboing Ivysaur as Marth is easy mode. She's floaty and has the 4th lowest horizontal air speed in the game so that makes her pretty much a stationary target, just handle the dair stall like you would handle Jiggs with only one double jump and you're set. Start with grabs at 0% (fthrow fthrow tipper fsmash for a quick 21%+edgeguard/ledgeguard situation) and just use whichever move pushes Ivysaur the closest to the ledge. She's extremely weak to fair chains, it's possible under certain % ranges to ken combo her even if she DI's away. Use your two DI mixups, utilt and dthrow, to mess up her DI faster than she can react and get free 30% combos out of it. Other than that it's very much the classic stuff : fthrow to set up a tech chase, and then you go ham.
Edgeguarding Ivysaur is also very easy mode.
Ivysaur will tend to shoot a razor leaf or a seed bomb at the ledge. There isn't much you can do about seed bomb (she regains control too early : if you edgehog her when she tethers the ledge, she can wait until your invincibility wears off and the seed falls to the ledge before pulling up), but it requires a jump to be done. And you can just time your edgehog against razor leaf, the only thing Ivy can do about it is to double jump and fair you after you've spent your ledge invincibility on razor leaf already. So basically, if she has a double jump, she can probably recover : jump offstage and put hitboxes out, just frighten her enough that she'll waste it and won't be able to use it to recover anymore. Better yet, bait those double jumps during your juggles and only go for the fsmash once she's spent it.
Then comes the edgehog part that you're probably familiar with.
If she strafes back, you can dair spike her, or bair to push her away from the ledge without a double jump or a dair, and with one less tether, so she's dead for all intents and purposes. If she strafes towards the stage, you have a couple options : either uair her as early as possible, so that you hit her before she touches the ground. She'll be easy as **** to juggle and edgeguard, from that situation onwards.
Second possibility, at 100ish%, do a reverse bair into tipper fsmash. ****'s free.
Third possibility, at early%, waveland onto the stage and grab her. It leads to the highest rewards in terms of damage, but gives up any chance of scoring an early KO, so don't always take that option it's not always appropriate.
Fourth option, RISING nair. If you do a falling nair with the same timing as when you're just trying to get back from the ledge against an edgeguarder, she'll be able to CC your nair. If you do it properly, even if Ivy strafes forward as much as she can,
she'll be sent flying backwards and will probably die. That's the most technical option but it is the most rewarding. Strafing forward is supposed to be Ivy telling you "okay you get a free hit but I get my recovery back", and since you're Marth you can just tell her "no". The con is that you have to do it as early as possible so you probably can't react to Ivy strafing backwards instead. It's a 50/50 in which either Ivy dies, or she's at the ledge and you're above the ledge : it's still a trade I would make.
Fifth option : waveland, eventually dash forward and pivot if Ivy strafed forward during your waveland, and fsmash backwards. It will be a guaranteed kill at high % but it's going to be your worse option early on.
There are probably more options but those are the main ones I'd train if I were you.
Note that if you don't feel like taking decisions (I know, fighting games are hard LOL), after about 50% a rising, advancing bair will cover every possible strafe routes Ivy can do. If she strafes back, she'll get tippered, if she strafes forward she'll get reverse baired and you can grab/utilt/fsmash her depending on her %.
You need to have a proper SDI to get out of Ivy's combos since half of her moves are multihits, but it's very possible. You can fair to shut down most of her most rewarding combos so that's very infuriating, but if you get predictable you'll get dtilted for it when you land. On the ground, most of Ivy's stuff is crouch cancellable.
To kill Ivysaur outside of edgeguards, it's usually not that hard. Your main setup is going to be side B. It's a 4 frames moves you have to CC, except most people won't CC it because you're using it at a percentage they're going to want to be using mostly survival DI at. From 90 to about 110 you can do side B 1, 2, and 3. The 2nd hit will miss, the 3 one is the killmove. After 110 you can side B into utilt instead.
If all else fails, spam fsmash. Yes, I'm serious. It beats full hop fair, one of Ivy's most common neutral game pokes, is safe on shield because it's Ivysaur we're talking about here, and you'll win the clanks with her ground moves because they're pretty much all multihits and there's not a single hitbox that deals more than 7% outside of smashes.
As far as CPs go, you want small platforms, or FD. The reason for it is that big platforms help her get out of juggles and tech chases, and you don't want that. So ban YI and Skyworld, Norfair/Dreamland if it's legal. If you do, she'll probably CP FoD, GHZ, and Smashville. She'll probably ban WW and YS, you can CP her to FD, BF, Lylat, PS2, ...
Basically just do Marth things and abuse the heck out of the stupid tether mechanics.
