I find pikachu to be a pretty unchallenging matchup.
The Chain Grab:
What is on every pikachu's mind when you're below about 50? The grab, that's the only thing they want and it's all they're going to try to do. Much like the Falco matchup what you need to do is platform camp with dairs and dsmashes until you are past chaingrab percents. Smashville is a great stage for this matchup because you can also throw waddle dees. Play as gay as possible, don't give up the grab for any reason. Get as much damage on pikachu as you can before you're finally past chaingrab %. You have 8 minutes, use as much time as you want.
Past grab %s, now what:
Get in the practice of powershielding pikachu's neutral B, it is very easy to powershield and you need to know how to do it if you're going to approach. Most pikachus jump when they use this move because of the lag on it. They will then land and try to grab you or hit you with a fair. However, if you space your approach right, you can do one of two things;
A. If the pikachu doesn't know you spaced your approach right, you can powershield their neutral B and grab them right as they land. Lots of your damage should come from grabs and gimping in this matchup.
B. If the pikachu is smart and knows you're trying to space their projectile, they will stop early and space their fairs on you. Play it out like fighting a marth. Throw your SH bairs into the air to stop this approach and ftilts out of shield if they hit you with one. My personal favorite is when the pikachu doesn't space the fair right when trying to go for fair to grab. Free grab for us.
I'm going to be honest with you guys, I've never fought Anther, so I can't say I've fought a true top level pikachu, but I've never struggled with pikachus. I usually live to very high % because they can't get the kill and at the same time, they die early. Just approach like you would approach ROB. I feel it's in our favor.