Yeah, so Marth/Lucina versus Pikachu is all about spacing.
If the swordsman can keep you out for a good portion of the match then they win.
If you can punish them hard enough after getting in as Pikachu, then you win.
It is very important for Pikachu to edgeguard Marcina offstage and it is important for Marcina to keep stage control.
If you want to be safe, try to force them into a bad position then trade an aerial with their up B, neutral special snipes can put them in a bad position and sometimes kill them if you catch their double jump. If you know what you're doing then just go straight for the read and hit them with a special or aerial move.
Marth/Lucina like battlefield while Pikachu doesn't gain as much on battlefield in Ultimate, so consider taking them somewhere else so that thunder and thunder jolt can be more effective. Lots of characters that have trouble landing will pick battlefield. Marcinas love to up B OOS to the afety of platforms.
Of course marcina has trouble landing so take advantage of that, keep them in disadvantage. Conversely, make sure you are not juggled by Marcina up airs; might need to mix up quick attack landings, b reverse, air dodge, use an aerial, or skull bash to offstage.
Also try not to get your shield broken, spot dodge or roll if shield is low. Heck you can even jump to escape more than you'd have thought possible.
Then figure out both of your habits. Make sure that you figure out and abuse his habits while avoiding auto-piloting with your tendencies. If you keep throwing the same stuff out, you will get bopped.
For example: You don't always need to be the aggressor. You can thunder jolt and quick attack camp and make him come to you if you're at the percent lead. Let's say your friend likes to dash away and pivot Fsmash you because you always chase after him. Instead, you can just use a grounded thunder jolt and grab him if he shields after the whiffed Fsmash.
Example 2: Marcina player loves to throw out aerial approaches and cover his whiffed attacks with Dtilt and Ftilt when landing. As a Pikachu, you may be content to shield the follow up attack and then punish with Nair OOS. But you feel that every once in awhile, the Marcina player will go for dancing blade or jab 2 and that always hits you. Sometimes he's even broken your shield with a shield breaker. In that case, you may want to roll away or behind your opponent sometime to keep him guessing. If he goes for the aforementioned dancing blade, shield breaker, or jabs, you will have time to punish, even with an up smash if you time your roll correctly. Then he will have to watch out for your rolls.
Figure out Pikachu's bread and butter combos and use them for good damage.
