Okay Moxin, don't take the criticism too harshly. I'm gonna try to help you out with some specific things you missed or could do better. It's only one vid, so you may have just missed it once and I don't care if you "normally land it", that's fine. I'm just going to throw out stuff that you missed in this particular vid and if it is something you're already aware of or working on, just dismiss it.
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You are honestly looking like you've established a decent foundation for yourself to get better, and that once/if you do play better opponents you may be able to adapt or improve your game accordingly. Your movement is fairly fast, but I'm sensing that a lot of your follow ups are things you do out of habit, not on reaction, because they work on this guy and you know it.
Particular stuff:
-When you're on stage you should be up throwing fox, even on dreamland. You can tech chase the entire platform with up air, and if they are directly underneath you, up tilt will still land. In general I think you should look for way more up tilts, because they will open up the amazing combos that you end up needing to beat a great fox. Chaingrabbing is something you can work on against the computer. Lvl 9 will DI vertically always and lvl 1 will DI sideways (tho predictably back and forth, but it at least gives you different DIs to play around with once you get to the point where you need to up tilt).
At 0:36 you want to make sure that you tip this up air. You up air'd too early. Work on short hop fast fall up air as the timing can seem a little awkward at first, but this is really really important for Marth.
At 0:40 you really want to get on to that top platform with a double jump waveland out of fox's up throw. You jumped a little late, didn't get on the platform, and now you're side Bing in the middle of the stage after using your jump. If you get knocked off stage at that point you have no DJ or side B - you're dead. Be very careful about how you use your DJ and side B and get your *** on the platform.
0:45 wreckless getting on the ledge. Wavedash off and fastfall on to the ledge. He could have running shined you for the kill. You also shouldn't be getting on the ledge there at all. At low levels, you can just destroy people with ledgehop aerials bc they just massacre people who aren't ready for them. At higher levels people will kill or at the very least hit you as you try to come up from the ledge much more often than you will hit them. Don't rely on the edge to give you an opening. Focus on your on stage game.
0:57 again going off stage for no reason, and wrecklessly
1:17 it's very tricky to ledgehop dair fox dependably here because there are different angles he can take that will alter the timing of it. Ledgehop and counter is the right move here, which you then follow up accordingly with offstage fair or ledgehog or whatever you read to be the right response to how he Up B's.
1:20 you get stuck in shield. work on wavedashing in and out of shield. You want to be able to transition fluidly from shield to movement and back whenever you please (whether its with wavedashes, short hop aerials, whatever).
1:32 whenever a fox jumps or better yet double jumps right above you like this you should ledgehop with up air. Follow up with up tilt, fsmash offstage, or grab (at really low%). Fox should be dead if he double jumps in front of you like this.
1:43 WALKING IS AMAZING IN MELEE. Notice how you dash danced in the wrong direction and then wavedashed in place in order to set up that edgeguard? Luckily for you he decided not to just regrab the ledge or side B, or else you wouldn't have had time. I also notice you dash danced in the wrong direction, which makes me think you might not be comfortable with starting your dash backwards yet. This is an insanely important tactic for Marth (aerial, dash away immediately is a staple high level Marth technique). Also don't jab his up B in that spot. Use dtilt as it makes it much harder for fox to come back.
1:46 so immediately following that jab you rush down there for maybe a reverse up B and get burned by fire. When fox up Bs this close to the stage, you need to stay on stage and counter his up B as he comes up.
1:57 always DI up throw
2:08 once you get a grab on fox facing away from the stage, you should fthrow to dtilt. It pushes them further from the stage and sets up better edgeguarding. You can run off fair on a good fox (that might work once, and then you'll start finding yourself getting killed for doing it). He can shine stall below stage, DJ into a wall jump back up, DJ regrab the ledge (he didn't do it properly) or DJ air dodge up. Then he'll be on stage and you'll be off stage, getting edgeguarded. Fthrow dtilt and then react.
2:23 again fthrow dtilt
2:31 it is really inexcusable to miss this tech, and you allow him to get a hit and potentially a major follow up combo. That would cost you half a stock or more against a good fox. You might not be very comfortable with GETTING hit. The swiftness of your offensive game may be because you are so focused on it. Once you get hit you need to be ready to DI, and properly, and also to not miss techs and recognize when you can escape and when not.
2:33 To top it off, after you allow him to up air, you knee-jerk react by double jumping and fairing at him. you never want to do this, do not waste your double jump, fairing in that position is predictable - he could just wait for it and shine you or combo you more now with no jump, which should be the end of you.
2:36 MORE OF THIS - dash away grab is too good - if you find that you up tilted slightly early and he's going to land on the ground before you can regrab, jab him and he will stand up automatically for you to then grab(called a jab reset).
2:41 Missed another up air tech chase on platform. Good fox does not give you many opportunities like this, and its critical to execute when you get them.
2:54 you should up tilt or fsmash out of this dash attack, not nair
2:56 you get stuck in shield again, this goes back to being comfortable defensively. Know the timings for shield stun, and wavedash out of shield with an fsmash or do any of the various short hop aerials that would have worked.
2:57 why run and grab here? you should just turn around in place and grab. This goes back to being comfortable standing/walking rather than just dashing for everything. This put you way way out of position (and probably is the reason you missed the grab).
3:00 again, I can not emphasize enough how this type of edgeguard will not work on a good fox. He has way too many options for you to commit off stage here, and more often than not you will find yourself being edgeguarded for it instead of the other way around. Stay on stage, jab/dtilt side B.
3:17 I will say that you have a good instinct to really go for blood once you've landed a backbreaking hit on fox.
In general tho, you are getting so SO much for free because this fox is terrible around the ledge (recovery, him on the ledge, you coming up from the ledge). Please don't take this too harshly, it's difficult to really explain how different this would go if you played a really solid fox.
Up tilt and try to combo into tipper fsmashes more.