alright guess its my turn
Again i play wayyy different then normal people so it might not work for everyone
Shiek vs Marth:
The main two moves i spam like crazy against marth is dash attack and grab. Downsmash doesnt work too well against marth, it can sometimes come in handy if they sidestep spam but its just better to predect the sidestep and then grab.
Dash attack: The big thing about this is to start the dash attack close to marth. If you do it from far away you will get shield grabbed, so do it so the dash attack goes through marth's shield. This move is good to do at any percent and can combo from mid-high percents.
Grab: When you get a grab always throw down. Very rare occasions do i throw marth back or forward even if im close to the ledge(im not good at fancy off the ledge edge guarding). When you throw down they will either DI away, DI neutral, DI up and away, or jump outta it. When they DI away grab them again. You have time to react to the throw so you can take your time and see which way they DI. So chaingrab if they DI away. Eventually when they are in killing range and they DI away go for the slap.
If they DI neutral i always go for the up smash. Depending on which way they DI it can be a regular or tipper up smash. Its good and it kills a low percents. Only a few marths know how to get outta the up smash(i did it mutiple times to m2k,cactuar,azen if that tells you anything) so dont be afraid to spam it. If it somehow stops working just go for the up air. Eventually at higher percents you will have to go for the up air anyway but spam the up smash as much as you can.
Ledgeguarding: It all really depends on how much time if you have to get over to the ledge. If you have time grab the ledge and do a back air. Time the back air so it hits right after they use their forward b to recover(this way you wont get hit by their forward b or get hit if they try to attack you as they are coming back), Marths will usually drop really low to avoid shiek's back air so just hang on the ledge and roll back on. I sometimes do the KDJ and do a neutral air from hanging on the ledge but sometimes it back fires so im a pu.ssy and dont do it. If they somehow make it on the stage and your still hanging on the ledge you can hit them with a fair, or even better is to come up with an up air and combo off of that.
If you dont have time to grab the ledge i usually spam downtilt and hopfully combo into an fair, very rarely do i pull a plank and come down with a fastfalled fair.
Coming back from the stage: Ledgeguarding shiek is the most common way marth kill(that and random as.s tippers). If you are near the edge and a marth down throws you first thing to do is NOT FREAK OUT. Most of the wins M2K gets is because he takes advantage of this. First thing you try to do is just see if your 2nd jump is enough to make it back on the stage(Marth will obviously try to hit you as you do this so either come back with an attack or air dodge though it). Also airdodging right away onto the stage sometimes throws them off. Otherwise you are gonna have to up b. Marth will be hanging there so you can either to 2 things: upb onto the stage or onto the ledge.
This is one of the times where you are gonna have to read your opponent. There are Marths who are patient (Husband, m2k, Cactuar) and those who constantly move (Lord Knight, Ken, DJ Combo). If it is a patient marth they are gonna hang onto that ledge untill the very last second so you have to up b back onto the stage. Constantly moving marths will usually get off the ledge so you can trick them and up b straight into the ledge.
Now if you up b back onto the stage the marth is gonna either fsmash you or grab you again depending on how much time they have. The best thing you can do is DI good and hope to get free.
General tactics:
One thing i cannot stand when sheiks approach marth is that they do it from the air DO NOT APPROACH FROM THE AIR Even if you are needle spamming you are gonna get buttf,ucked. Marth's up tilt is too good and can easily combo into nasty stuff and usually your attacks will get shield grabbed and your get screwed that way. So i stay on the ground and go for dash attacks and grabs.
When you do get grabbed by marth they will usually only throw down, forward, or up (only person who throws back is ken and i dont understand how he combos with it) If marth throws down always tech away unless you run outta room on the stage cause marth will go for the tipper fsmash. If they throw forward you can automatically shield at low percent but again tech away. For the up throw always DI Up and away and spam jump. You will not get hit by anything but you have to be careful on how you get down cause you dont have your 2nd jump anymore and can get combo'd easily.
If you do get a marth that just camps the ledge waiting for you to come to them like gay as.s M2K, DONT GO OVER THERE. If they are gonna play gay, play gay back. Just spam needles and let them come to you even if it means all 8 mins go by.
Think thats all i got for now. This thread is awsome btw, glad to see everyone contributing. Merry Christmas!