DI out of Dsmash (rotate your directional stick in quarter circles up, and mash the c-stick up, you'll pop out after taking about 3-5%)
Usmash is a killer, as is nair (unfortunately for Pikas, they tend to stale their nair, leaving that out of the mix
)
fsmash is very easy to avoid, not to mention it must be sweetspotted (the part closest to Pika's face) in order to have kill power.
Dtilt can trip.
Jab is pretty useless against you (assuming you have a naner in hand) and can almost always be retaliated against.
9 times out of 10 your Diddy hump/kick will go over him. Pikachu is gay.
You can not SHfair a grounded pikachu. It goes right over him.
QaC is most easily interrupted by Diddy Kick, which at worst will either miss or trade hits (you take 1-2, he takes 9-13, good trade
)
His trip animation is FOREVER. Seriously, naner locking a pika across the stage is extremely easy to do, even if you're not glidetossing.
Bair can be DI'd out of.
His glidetoss is good, but also difficult for him to space properly, because if he has spaced it properly, odds are you have enough time to *insert option here* on reaction, plus other players tend to be **** predictable with our naners XD
Tjolt will gimp you if he catches your barrels offstage, as has been stated be careful when recovering.
When Pikachu is hit by his own thunder, he has invincibility frames, so be careful about punishing his downB, if at all. Sometimes it's better to just recollect yourself instead of going for the punish.
Most pikachus have a pattern to their QaC, so if you can watch your opponent while he's playing someone else and study it, good, if not then try and learn it midgame
Things to watch out for:
Tjolt spam (powershield, SH > naner is really good for punishing the camp)
fthrow > usmash (unavoidable at low percents, don't get grabbed!)
thunder (slowwwww BUT if he uses it and you're above him, it comes out faster than you'd think)
Autocanceled dairs, second hit dairs (it has a hitbox when it hits the stage)
fair (DI out!)
dsmash (DI out!)
bair (again, DI out)
dair (shield it, fair can beat it while trading sometimes, bair always beats it if spaced properly)
Things we can do:
naner locking made even easier
his sideB can be beaten out by bair, or sometimes spaced into a dair (while he's recovering)
Diddy Kick the QaC
Diddy Kick through Tjolt spam
hardcore punishment on jabs
DI out of dsmash > punish
fair eats through tjolt/his aerials
QaC has LAG at the end of his QaC spamming (once he touches the ground and stays there) ABUSE IT!
Good luck