sigh... nowadays people think its all about "shffling and mindgames and techskill"
ok basically your problem is tactics.
for example, if you know that shiek is going to needle you from afara, and you even manage to "goad shiek into doing it" and then you can perform perfect SHDFairs whilst approaching - that hasnt really put you into a better position at all if youre still on the other side of the stage whilst shiek is far away enough to throw needles are you without worry of you chasing her.
to deal with dash attack there are several things you can do. just being able to shffl isnt one of them
most common is shieldgrab - from here you can fthrow and mix it up with your nice long range frontal attacks <try to avoid fsmash unless youre going for the kill, but forward+b, fair, nair, dtilt, ftilt, utilt all work>
other than that you can throw off the edge <use downthrow for throwing behind>
backthrow can be common in that players will always attempt to jump out of a backthrow <so you can follow this up with an anti air more - utilt, uair, jump forward bair, etc.etc. - even use forward +b to gain some additional space and stun - and in the air you can use neutral b as a VERY long ranged weak finisher>
uthrow is good for your basic juggles <uair usually>
other options are crouching at low percentages - meaning that there is little to no knockback. this usually means you can attack straight away - fsmash might not be best - but ive seen you can use low knockbacks like dtilt, UTILT especially, shffl nair, fair, uair if youre good - and if youre good you can combo this into more frontal attacks or grabs or juggles
at very high percentages a rare tactic is to shield and up+b straight out of shield <sweetspot is right next to you>
<ken loves this move>
if hit right you can either do it reverse OR forward and it will sweetspot and have a decent knockback
other options are to shield and shorthop dair out of it, sometimes you can get away with the ther airs.
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basically, theres plenty to do against dash attack which is much easier than "trying to shffl a timed fair so it overprioritizes her" - they both end up with the same result, one is easier to do. heck if yuore not good at aerial combos, a grab may be much more sufficient.
if you REALLY dont like standing still you can dashdance back, wavedash back, WALK back, jump over and wait for cooldown, use dtilt as shes coming.
DONT be tempted to use counter - the ONLY situation to use counter is when you are playing NTSC and get downthrow - dash attack at 30~% and jumping isnt fast enough and shes too close when she dashes for you to fair.
whenever you can counte,r you cna do something else.
dash attack is one of her laggier moves, and one which is much easier to space from, so you can pick out a nice long drawn combo which youve memorised or can make up on the spot once she does one.
-other problems with shiek are grab, which are usually done DAJC or JC, which the response is usually jump and attack with dair, or WD/dash back and wait for the lag.
to deal with dsmash - always DI up vs shiek and dont be tempted to techroll all the time into stupid places - getup attacks are good SOMETIMES aswell as a normal getup to shield or dodge if she goes for a grab
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other ways of dealing with it are spot dodging aswell..
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ok yuo get it - there are infinite possibilities. your problem is youre picking the wrong tactic to deal with it. shffled fair isnt going to help you THAT much - go for a grab, or a dash back + free attack.
also - if youre not at a position where if you get a chance to attack, and you dont do a HUGE, soul destroying unbeatable combo on her - then your marth needs work.
against the best marths its like, you make a mistake, and youre taking 4 fairs and a finishing move in the face because he combos you, and if youre % is high you get instant tippered.