It's really difficult to tell people how to fight Pika. Pika's whole schtik is that:
a) Pika harrasses everyone without an absorber with thunderjolt, but from mid-air at a far enough distance that you can't hit him, and from an angle where reflectors don't threaten him, forcing most characters to approach Pika on his terms.
b) Pika attacks primarily though disjointed auto-cancelled aerials, meaning you can't simply look for certain attacks to punish, but have to look for landing punishes and box Pika out in the air with superior range or angles.
c) Pika's recovery is so incredible that he effortlessly dodges standard edgeguarding and juggling situations, meaning you have to preemptively call out the path of his Quick Attack and stick out a big meaty attack to intercept him, or block his attempt to QA into you and punish the short recovery.
This means that a) you have to approach Pika because he has no interest in approaching you in a neutral situation, b) you have to be comfortable with your options in the air, because you have to meet Pika in the air and box him out, and c) you have to have unique counters planned for a Pika attempting to QA over or through you.
Lika @
Antonykun
said, this is all really specific stuff you'd have to ask the Pit boards for the best options.
