What to do

What not to do

Don't edgeguard him like Sheik by grabbing the ledge and punishing his landing on stage. Pikachu doesn't have enough landing lag for that.

Don't dtilt or fsmash at the edge so that your sword barely sticks out over the edge. You have to be teetering or close to teetering to cover Pikachu's options.

Don't grab ledge and do a ledgehop aerial like bair or uair.

I have had opinions about this for a long time, and I finally did the research to figure this out. Marths traditionally have had a lot of trouble against Pikachu, and one of the reasons for this is that they do not know how to edgeguard Pikachu. Marth should not struggle that hard to edgeguard Pikachu. He has strong options to make Pikachu's life hell.You can cover every up-b angle to the ledge with dtilt. If Marth teeters on the edge and dtilts, it covers every angle Pikachu can approach the ledge from. It covers the high angles as well as Pikachu's lowest sweetspot angle (in this picture, Pikachu will grab the ledge on the next frame). For the record, Axe tends to angle his up-bs higher than this. Furthermore, you can react to Pikachu's up-b with your dtilt. On the first frame of Pikachu's up-b, little white clouds appear below him. The reaction window is about 2 frames easier than reacting to tech in place with grab. This only works if Pikachu will travel the second leg of his up-b. If he only travels the first, you cannot react with dtilt.In this way, you can cover all of Pikachu's attempts to grab the ledge. If Pikachu tries to up-b onto the stage, you can cover those options easily too. If he hits you with his up-b, there is so litle hitstun that you can instantly punish him. If he lands on a side platform, he has 24 frames of landing lag, plus hang-time at the end of his up-b. This is enough time to hit him with a uair, utilt, fair, or fsmash depending on the spacing.Marth has other options as well. Forward smash will also cover all of Pikachu's angles to the ledge, but you cannot use it on reaction. Often, Pikachu will up-b at a predictable time making forward smash easier to hit. You can see PPMD make a lot of good use of this in his set versus Axe at Smash Summit 1 until Axe starts mixing up his up-b timings to make PPMD miss. You can also see PPMD use dair to cover Axe's initially predictable up-bs.Another thing to note about Pikachu's up-b is that his hurtbox expands massively for 1 frame at the midpoint of his up-b. You can see MacD abuse this fact in this image . MacD probably could have been farther away honestly. And then Marth's fsmash reaches WAY farther than Peach's bair. When teetering on the edge, you can often hit Pikchu in the midpoint of his up-b with an fsmash or jump off and dair. PPMD does both of these in his set versus Axe.With this knowledge, I hope that the Pikachu matchup can be easier for Marth mains. Effective edgeguarding techniques will also improve the effectiveness of uthrow juggles. Marth has a guaranteed uthrow->uair on Pikachu that leads into a strong juggle. However, this is often cut short by Pikachu DIing offstage and recovering for free against clueless Marths. With better edgeguarding, DIing offstage may not be as appealing an option.