Alright Marshallowman
Welcome to the Competetive side of SSB :D Hope you enjoy it.
Ok you've only been competetive for a few days so the things I list I do not expect to be fixed easily at all. And if you do fix them easily, then wow good job.
1) When you use a certain move, and you miss, do not try it over and over. This makes it predictable. For example, you used Side B a lot (which is good, because that is one of Marth's best attacks), but if you keep repeating it over and over, they will catch on to what you're doing.
2) Work on your timing with the attacks.
3) Work on spacing. (Spacing is keeping a constant distance away from your opponent so that you can hit him but he can't hit you).
4) Try to use more short hops rather than full hops. Short hops keep you closer to the ground, and it allows you to use your aerials quicker.
5) When you're above an opponent, do not DI (directional influence, which means tilting your control stick to move your character while airborne) into the opponent and then Dair. That's way too predictable, especially how you did that most of the time you were above him. Marth's Dair is very inefficient, even for spiking, and should never be used unless you are playing at a high competetive level.
However, if you do want to try to hit them when you're above, try to DI to the side of the opponent and fair them. (Fair stands for Forward aerial attack).
6) Try to use some more air dodges when trying to land back on the stage.
7) When knocked up and away, and you're recovering to the stage, do not use your double jump immediately. Save it for when you get closer to the stage if you need it.
8) Less fsmash.
9) Most of the times when you used fsmash, a fair or side B would have been better.
10) When they're close and shielding, replace your Side Bs with dtilts (stands for down tilt, which is the attack where Marth crouches on the ground and stabs you with his sword on the ground).
11) Replace some usmashes with utilts or uairs, as usmashes are slow and can be punished.
12) Do not use Dolphin Slash (Up B) in the air to hit an opponent. If you miss, it creates and opening for the opponent to attack you. Even if you do hit, most of the time they will not fly far away enough, and instead they will be close enough to you to punish you. Instead of using DS, use a uair or other aerial attack.
13) Don't roll dodge as much. Try some side dodges, and even walking or dashing away from an opponent works! (To new players especially, this may seem very dangerous but if you try it a few times and you may see how good it can be).
Basically, most of the things I listed here can be fixed with more practise, considering you've only been competetive for a few days
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Oh, and welcome to the Marth Boards!