Helpful hint if you're getting nailed by Pikachu's famous "high priority" down smash, you can smash DI up out of it..the move becomes nearly harmless if you smash out of it early.
Yeah, you don't necessarily have to smash DI. After the first hit, press up on the control stick and it'll harmlessly toss you aside doing about 3% dmg. But if you do it too late, you'll take some more damage and throw you aside and they'll be finished with the attack, instead of being placed RIGHT NEXT to the pika. My team mate was a pikachu and whenever everyone was caught in his downsmash, I often found my self out of it instantly, and was able to grab one of our opponents while they were being flung around in it.
Pikachu is going to punish your rolls, and if you try to land with an air attack. Avoid rolling toward pika, or even when near it. Sidestep and downsmash are their favorite things to do. Instead of trying to shield through it, try the powershield and grab him. And yea, the waddle dee toss is one of their worst fears. Mainly because when he's shocking someone, the entire attack slows down, giving you enough time to charge up a f smash or, to be flashy, a jet hammer. Another thing you can do, as an approach, they'll see you running, and be prepared for an attack with a sidestep down smash. Dash attack :D. The delay of it is exactly = to that of pika's side step, and they won't get enough time to dsmash.
And grabbing will help keep it off the ground to avoid down smashes.
I play against a pikachu main, and a marth main, and I had that same problem @_@