For about a day or so I had a lot of trouble with pikachu. Its dsmash, really. All you have to do is remember the amount of hits it yields. If pikachu comes near you and is about to do a dsmash, shield the entirety of the hit (it will not break a full shield) which is around 5 hits I think, and then shield grab into whatever. Pikachu's dsmash is one of the most broken smashes in the game; rolling away will not work because there is a good chance its dsmash will catch the vulnerability frame as you roll, then you are screwed. Just shielding the entire hit is your best bet. A good tactic is purposely jumping high over pikachu do the only thing they can do is thunder, then reflect. If you do it enough times, the pikachu will be afraid to thunder; so you can jump high in, like you're going to reflect, but then fall onto it with a dair or bair. Also, remember: spiking is your friend, especially on a pikachu.