I'm pretty good with Kirby. Pichu's just another fun addition to my mysterious field of smashing abilities.
Pichu might be weak, he might have low range, he might have a tendancy to get *****, but it's worth it. Pichu is too fun to pass up. I personally don't play Pichu as much, but I'll tell you some things I picked up playing as him.
=Approaching=
I usually run into a Full Jump, as I'm in midair I would use Neutral B. With the lightning attack rolling across the ground, I would chase that up towards my opponent. If I try to SHFFL, I won't get shield grabbed because the neutral B covered for me.
Otherwise, all you have are SHFFLs. Pichu's ground attack range are terrible so you won't really be approaching with that. Pichu doesn't have more than that. His grab is pretty short too.
=Positions=
Competetively one would trade hits. Pichu isn't one of those. Pichu has a more complex way of play where one has to play smart. You're constantly having to out smart your opponent for every move you make. For if you make a mistake, you're going to be punished terribly.
There is one type of positioning called camping. It is basically running away or keeping a distance from your opponent. Usually you're doing that to lure your opponents into your "Pichu Traps" thus again comes the idea of playing a smart Pichu. Ideas of these would be "dash dancing", ledge stalling or using projectiles. Once the trap is activated, the opponent is lured, the damage was taken on the opponent you repeat.
There's another position to go into taking is called spacing. This is similar to camping except you're in a more aggressive state. You would be dash dancing back and forth and there will be possibilities of you attacking anytime or your opponent attacking you every time. This is an effective position because you will be keeping a distance from attacking, but close enough to attack your opponent. It's basically trading hits without actually getting hit (or hit to smaller extent) your self. Don't forget, mistakes will always be made, so play smart. xD
=Edge Game=
There will be times when your opponent would be at a relatively high damage, and you just don't know how to KO them. You're in the situations where you need to find a way to get your opponent off the stage. Some options are "hitting your opponents off and KOing them" or "throw them off the stage, not KOing them, and then guard them from the ledge." Pichu has non more than his smashes to KO, so he usually is edgeguarding.
Some good attacks to use when you want to throw your opponent off the stage include: his down tilt, side tilt, back throw, front throw, forward smash, down smash etc.
But how? These attacks have so little range. I talked about spacing and waiting for the moment your opponent makes that mistake where you would charge in and punish them. With this technique, throw your opponents off, and it's just a matter of guarding the edge.
What're some good edge guarding technique? It's whatever your imagination wants it to be. Or at least anything that works. Apparently Pichu's ground attacks have little range, so I would recommend of stage edge guard.
Some edgeguarding techniques are:
Tap A
Down A
Forward Smash
Down B
Shooting a Neutral B out
Jumping off and using a down-air (or any other aerials)
=Off Topic=
My Pichu sucks, but I hope the information worked.
=EDIT=
I forgot you mentioned help on a Marth. I already basically explained it, but some things to watch out for. Remember, you're the one with the projectile, so he should be the one approaching. Lure him into your traps and don't make SHFFLing too occasional. His neutral B does 1 damage to himself, but it's for a great use. Watch out for that Tipper!!!
