Sorry I forgot about that critique, but I'll do it now.
I'll do the match on FD right now (
http://www.youtube.com/watch?v=G2uAob-t7yA). If you want me to do another, I guess I can, but I'm not sure how helpful it will be....
0:03- Leading off with a Nair that wasn't spaced is already a bad idea. Only Nair like that if you're sure you're going to hit him, otherwise he can just hit the ground and easily hop over you or punish your landing lag. Fox wants you to wiff your spacing so that he can rush in and punish it. Your job is to space him or grab him, and mix in DDs and aerials/Dtilts so that he doesn't know what's coming next.
0:04 ish- Don't Dtilt after a Nair like that, since most Foxs will just jump away. A good move would have been to DD and see what Fox wanted to do. In this case, you would have gotten your grab because a FJ FF Dair is easy to punish.
0:10- I'm seeing Nair to Dtilt again. Karn isn't really running in for grabs, so having a constant hitbox out is not necessarily the best idea. If Fox just camps you like that in your face, then go for a few long dashes to Dtilt or grab or retreating Fair and reatc accordingly.
0:14- Well, you know what you need to work on there. Figured I should point it out anyway.
0:17- Again with the Nair. It's not even very good against Fox unless you're right where Fox's SH Nair will land. If the Fair didn't connect, then you should DD in and see what Fox's reaction will be.
0:25- Go for one Uair to continue the combo. It helps.
0:30- Karn really isn't jumping OOS quickly this match, so you would be safe with a grab (which is what you should prefer here anyway). Try to do it quickly if you can. The longer Fox has to think, the worse things are for you.
0:35- Again, not a terrible idea, but the walking up instead of WD'ing around did kind of give you away. Try to mix up your approaches.
0:36-38- make sure you space properly and DI away from Fox's jab. Uair would have been ok in this situation as well. Also, you want to stay low to the ground with Marth vs everyone since Marth is extremely weak from below.
0:41- I guess you got nervous? You can jab reset Krn so he stands up in place, or Fsmash him, or tech chase grab/Fsmash.
0:43- just WD OOS. You could even get a tippered Fsmash off here I bet.
0:46- You should've AD'd earlier so that, if Fox had done the Bair, he would have missed you. You did it too late and got Fsmashed.
0:52- WHAT. Counter is the last thing you should have done there. That is every Marth's dream right there. You could Fair him, probably turnaround Dair him, first hit of Dancing Blade to Fair him, Bair to back on stage set up for more edgeguarding him, and probably more. Counter only if Fox/Falco are recovering low.
0:57- You started it right, but finished it wrong. There's no follow-up for that situation other than to chase Fox and see what he does. Actually, re-watching that part, if you had walked a bit farther you could have gotten that tippered Fsmash off after all. Just be a bit more patient with your Fsmashes, and make sure you're in range, and you should be set.
1:00- Dtilt wasn't good there. Either dash back to see if he aerialed or retreating Fair'd to cover the most options. Fortunately, Karn AD'd so you got the grab anyway.
1:02- I think Bthrow to Fsmash works at this point.
1:04- Dtilt would've been great there.
1:05- That's a really risky move if Karn had jumped back onto the stage instead of rolled. Stay grounded and Dtilt. If he attacks, he gets Dtilted. If he stands up, he gets Dtilted. If he rolls, you have time to turn around and grab him. If he jumps, you can Fair/Uair chase him.
1:12- You can always grab the edge so he's forced to firefox, you know. =p
1:23- Starting at 21% on the Uthrow CG on Fox, Marth has to DD grab.
1:27- Ftilt is almost always a bad idea vs Fox, and it's no exception here. You're walking up and expecting Karn to not know what's coming. You need to be more unpredictable with your approaches. The best way to do this is vary your DDs: this includes the length of the DD and how quickly you do it. It obviously also matters what you do out of the DD, but the DD itself is vital to Marth's effective approach/baiting game. Remember, against aggro Foxs you'll want to DD more so you can land grabs after they Nair toward you and other things like that.
1:42- Wait for the invincibility to wear off. It was a good SHFFL though. =p
1:45- You gotta follow up your hits. You had him stunned in front of you, don't be afraid of him counterattacking. Go for a grab or dash after him and see what he does. When someone is hit, they want to get away from the pressure, not try to attack back into it. You have to remember that.
1:49- 4th Dtilt was obviously a bad idea. I think a grab would have been better here, but pushing Fox offstage is also a good idea.
1:50- Make sure you DI both the throw and the Uair so you can live longer/take less damage. DI the throw to one side, then DI the first hit of Fox's Uair to the other. Be sure to smash DI (move the stick when the hit connects) when the first Uair hit connects, or it won't work.
1:58- Uthrow would've been better there obviously, but you followed it up with a good read, s it doesn't really matter.
2:00- I think you might've had time to WD Utilt or at least Uair Utilt or grab.
2:01- Not a bad idea, but most Foxs tend to spotdodge after they've been CG'd. The best way I've seen to punish this is to do a long part of a DD towards them, forcing the spotdodge, then quickly DD back and towards them again so you can get the grab. Works really well for M2K.
2:10- Too many pokes. Go for more grabs once you have him cornered.
2:12- I know it's hard, but you have to try not to roll much vs Fox. Retreating Nair and Fair and WD back OOS are your friends.
2:30- I've talked about the first mistake already, but that dash attack follow up wasn't a good idea there. Now if you had missed the Fair, then it probably would've worked well, but since you hit his shield you can predict that the Fox will either hold his shield, roll, or jump out. Karn has been jumping OOS so much this match, so it might have helped you here to do an empty SH and punish with either a Fair or Uair.
2:37- Make sure you JC when you CG. It helps then and at other times too.
2:49- The last hit wasn't a good idea, since you aren't playing someone who jumps in quickly. It's ok to let the DB finish before the 4th hit. In fact, it trips people up.
2:52- Not a terrible idea here, but it's kinda risky. WD OOS would have been good because you can turnaround and grab Fox or react to his next action better that way. Also, most Foxs would probably go for grabs there, so WD'ing back might net you a free grab or Fsmash.
2:56- Dtilt or Fsmash or even a jab reset would have been good here. Don't just let his tech go unpunished.
3:05- I'm not sure if that's an AD or a WL, but neither of those are a good idea in that direction at that point.
3:16- You didn't move after that bad decision to DJ. At least shield, roll, or retreating aerial. You don't want Fox edgeguarding you too much because he's not bad at it.
3:20- Wrong aerial?
3:26- Good ol Nair problem again. If you're on top of Fox, either wait out his spotdodge, or grab him. I think you might've been trying to intercept his jump OOS, but Foxs usually won't jump if you're THAT close, which is why you gotta have your spacing down and you have to watch your opponent.
3:33- Dtilt was an ok option, but grab would've worked. That second Dtilt is obviously not a good idea though. I think I'll just tell you that you should only Dtilt an opponent's shield once and then react to what they do: they shield, you grab. They spotdodge, you grab/Fsmash. They roll, you chase. They jump, you chase.
3:38- Try to hold that Fsmash.
3:40- Bthrow? Dthrow might have worked, and Uthrow is always a good option vs Fox. You can follow up with a Nair at this point and it'll have a decent amount of knockback so you can chase Fox to the other side of FD and still pressure him.
3:44- You're getting angsty here now. You know you have to wait him out and react to his movements.
I'm going to stop here since I've given you a lot to look at/you get really nervous after this point. Notice that you've done some of these errors more than once in this match and I may have only pointed them out once to you. Treat all of my critiques equally and learn from them. I promise they will help you improve greatly.