Well, you should work on MCing so you can vary the height of the missile - it's quite possible to fire missiles that hug the ground. I used to play a Sheik that ducked bloody everything and forced me to get that down consistently.
If/when you get a charge shot - save it for when you see the 'chu thundershocking. The 'shock is a laggy enough move that you should be able to do this without him being able to move/shield in time. Your shot will easily go through it (and will hit him when he's low).
If you don't have the charge shot, I wouldn't recommend playing too defensively - I usually resign the range-spam game when I find myself being outdone.
Pika's moves don't have the kind of range your moves generally do - take advantage of this. Stay within your fsmash/ftilt range, but out of the range of his moves (baring his fsmash - hopefully you can move during its slow-ish startup time).
Don't shield the air attacks. Your fsmash should go through pretty much anything pika can do in the air; intercept him with them. You may need to wavedash back and fsmash when you see him approaching with an aerial, but I've found I can fsmash in these situations without having to wavedash back too often.
If you find yourself in the air, try to have your back to him - your bair works pretty effectively against pika (when he is also in the air - doesn't work well if pika is on the ground). Otherwise your fair's disjointed-ness should outdue anything pika can do in the air. In fact, you can probably just intercept his aerial attacks by jumping into them with the fair.
If you're both on the ground, use your range - ftilts, fsmashes and dsmashes. Combine these with wavedashing to bait/confuse him.
If he 'shocks on the ground up close enough you can SHFFL a dair on him thanks to how slow the 'shock is.
In this matchup I only really use the nair thanks to it's speed, preferring the priority/range of the fair and bair.
If you can get a good feel for how he uses his up-b while recovering, I believe you can get in the way with the nair. So if he's coming up from below, jump just off the edge with the nair hoping he'll quickattack into it. I *could* be wrong that this works and being thinking of another character, but its worth giving a try. Otherwise pika's **** good recovery can be hard to deal with.
Abuse your range and priority and you'll find this can be a pretty hard match for a pika. Just gotta get a good feel for it.
embarrass big damage...