Sheik can punish and techchase Marth like no other. If you space well, she doesn't really have any good way to engage on you. So stuff her approaches with d-tilt and situationally f-tilt. If she tries jumping over your d-tilt, up-tilt and juggle up-airs. You constantly want to be aware of how far her dash attack can go and space yourself accordingly. If you see her waste her double jump, don't let her touch the ground again, Marth can juggle Sheik fairly easily, and Sheik doesn't really have a good way to get back to the ground. Random yolo fsmashes will get you punished everytime, so only use them if you're sure they'll hit. Don't get too cocky when edgeguarding either, she's invincible during a large portion of her up-b, and getting hit by it can easily turn an edgeguarding situation for you into a techchase/ edgeguard for her. And finally, make sure you're saving your double jump and recovering low, because she'll try to fair you every time you recover high. If you have no choice, be ready with a fair or side b to stuff her approach. A lot of it is just practice though man, just try to keep some up this in mind the next time you're playing. Good luck!
