I have a tournament tomorrow, been playing Ivysaur since 2.6 and feel pretty confident with him, but my MU knowledge isn't great.
So far, i'm pretty sure i'm gonna have to fight against good players in a few of these MUs and would loooove a quick summary on them.
Snake
DK
Luigi
Shiek (This one I need advice on especially, it's one of my most inexperienced MUs and will probably be the most common MU I face)
Marth
Fox
Falco
Sonic
Ganondorf
Snake is a fairly straightforward Matchup. You have to be paying attention to where he plants his mines, so you can blow them up with Razor Leaf. Razor Leaf is also great at keeping control of grenades. At low percents, take every opportunity you can to get a grab, you can get amazing damage and solar beam charge on him out of grabs (I'm talking 50+ damage, depending on their DI). Snake is a character with great stage control, but Ivysaur has that too. At mid percents keep him pinned down in a small place with Razor Leaf and Seed Bomb, and when he's vulnerable go for a Fair or Nair to start a combo. Practice juggling with Seed Bomb as well, Seed Bomb and Upair make Ivysaur comparable to Brawl MK or Melee Marth when it comes to Juggling, and if you do it right you can cover literally every option they have when they're above you (including trapping them into a position where you can kill them with Solar Beam and Razor Leaf).
DK is very similar to the Snake Matchup, except you don't have to worry about grenades, mines and other projectiles. It is the easiest matchup in the game (imo). Bombard him with Seed Bombs and Razor Leafs to force him into shield, and then grab him and proceed to do an easy 80% (usually getting Solar Beam charged and finishing off with that. In this matchup you really just need to outspace him with Razor Leaf and Fair, and make sure you have your setups into Solar Beam and Vine Whip down. He punished really hard though, so be careful about making mistakes.
I don't really have a lot of experience with Luigi, so I can't help you there.
Sheik is one of my worse Matchups, all I can tell you to do is watch out for Needles, as they demolish Razor Leaf, and when recovering, try and avoid slow falling Nairs and Bairs, as they can really screw up tether recoveries.
Marth is one of Ivysaur's tougher Matchups, but it's definitely possible. Marth has a lot of range, which makes one of Ivysaur's best traits less effective. He can also edgeguard you really well if he knows how, but few Marths do so I wouldn't worry too much about that. Make good use of the Belly hitbox on Upair to demolish his one dimensional recovery (will usually trade, so there isn't too much risk of SDs).
Fox is another character that can gimp Ivysaur really well. Shine is really deadly offstage, so if you see them jumping off to kill you with it, try and hit them with Fair before they get a chance, and then recover before they can so you can edgeguard them. Fox is extremely combo-able, so make use of that, as well as getting really easy gimps on him.
Falco, like Fox, gets combo'd really well and can be easily gimped. He can't gimp us as easily as Fox can, but he can edgeguard us well if the player knows how (they rarely do though). If they decide to play it defensive, the matchup becomes much more difficult, as Lasers are much better than Razor Leaf and are much more difficult to deal with.
Sonic. Sonic Sonic Sonic Sonic Sonic. DO. NOT. GET. SENT. OFF. STAGE. EVER. Sonic's Spring gimps Ivysaur with no effort whatsoever. If you get sent offstage, and they know that, you are dead. However, because of how much he was nerfed in 2.6, Sonic is no longer impossible to deal with. You can keep him out with Razor Leaf and Fair, while also randomly shooting Seed Bombs to try and limit his movement.
Ganondorf is another character in the vein of Donkey Kong and Snake, in that they get combod to hell and back by Ivysaur, but have strong punishes that don't make the matchup impossible for them. The main thing to watch out for with Ganondorf is Flame Choke. You can stop Flame Choke with pretty much any disjointed move, I prefer using Razor Leaf and Ftilt, but Fair and Bair work really well to. The biggest thing to keep in mind is Aerial Flame Choke. If you don't tech it, he gets a free Fsmash. This is a problem, because it's a very deceptive move and difficult to tech. When he grabs you, he slams you to the ground
, and then you bounce up, and hit the ground a second time. Make sure you tech the SECOND time you hit the ground, not the first.
Overall, the main thing to keep in mind in ALL of these matchups is that Razor Leaf is a pain in the ass for the other character to deal with, unless they're Sheik with a full thing of Needles. Make sure you are effective with Throw Combos and Upair chains, and practice Upair gimps as well. Seed Bomb is an extremely useful, but criminally underused move. Make use of it.
Good Luck Tomorrow Dabuz! Is the tournament Crossfire?