Pikachu really only has about 6 moves you need to worry about:
Fair - His main method of approach. High priority and long duration, good at poking shields, but rather short range. Not sure, but I think you can outrange him with an Ftilt, and I'm pretty sure you can counteract with an Utilt or Dsmash.
Dsmash - His best defensive move. Learn to DI it and you basically take away its power. Don't try to shield or dodge it though, as it is long enough to poke/hit you out of the dodge. Keep your distance, backtrack if he seems about to use it, and then hit him in the lag.
Thunder - Too laggy to use unless you're in the air, but it can be deadly if you are. Try to avoid being directly above pika at the top of the screen, because lightning hits very quickly up there and kills at stupid low percentages. Use DI to your advantage. He may also try to use this to edge-guard. If it looks like he's trying it, either airdodge through it if you're too far in, or back up, drop down, and tether the ledge as fast as possible.
Grab - He will chaingrab you if he knows what he's doing, and there's very little you can do about it. His grab range is bad though, so space well and avoid laggier moves (Upsmash, Up air near the ground, Fsmash, Dash attack) unless you know they will hit and knock him away. Throw out single jabs a lot to disrupt potential grabbing.
Thundershock - Just learn to jump it and you should be fine.
Fsmash - I find this annoying because if the attack starts at all, it will hit you even if you hit him. Be cautious about it, and don't jump to punish if you see him starting it.
Good moves for you to use are as follows:
Uptilt - Always a good choice. Prevents basically any aerial shenanigans and works great against dodgers.
Downsmash - Again, a great defensive move. Good against dodges and shields, hard to punish, decent kill power
Fair - Quick, allowing you to hit Pika out of most of his aerials.
Bair - Longest ranged aerial. Abuse the crap out of this for approaches and such
Needles - Your projectile game is better than his. Make him know this.
Ftilt - He's not the most lockable of characters, but you can still combo well with this. I'm a fan of Ftilt -> Ftilt -> Utilt -> Aerial follow-up. Or at lower percentages you can even walk into more Ftilts.
