Pikachu's moves can be split up, as with any character's moves, into Short and Long Range moves.
Long Range:
Fortunately, Pikachu has very few long ranges.
>B - Zelda doesn't have a counter, so just move away from him, prepare to dodge, then smash. Or, if possible, Din his *** while he charges.
Neutral B. This move is easily deflected with Nayru's Love. There is a lag while he does the move, so, if you're close by, dodge towards him and smash.
I'm going to ignore ^B since it's not a strong attack. Just prepare to USmash if he gets close.
Short range attacks:
Down-B - Zelda can deflect this back at him with NL. The hitbox is tiny, but if you think you can't get NL in time, just dodge away. Better safe then sorry.
Smashes: Pikachu's Smashes are strong and quick. It's best to make him come at you, in my opinion. Just dodge JUST out of the way, and when he tries to get you again, prepare a throw, F-Smash or NL. It's a good time to play mindgames when he's just C-sticking everything.
Aerials: Pikachu's air-game isn't particularly his best asset. Fortunately, Zelda's is. If you can get him, and keep him, in the air, you've got the advantage. From what I've seen, Zelda always wants to be under. (No dirty humour intended.) When he's in the air, he will either do a couple of things. We've already discussed the Thunder attack. Just FW beneath him, (practice the 16 directions. They are incredibly useful) and then U-tilt or U-Smash.
Keep in mind, every player has their own style. A lot of people will encourage you to be more Din's fire based. I happen to be FW based. That's alright though. Whatever works for you...
To summarise; Deflect "projectiles", keep your distance. Make sure he's aware of Din''s capabilities, and wait for him to get close. Dodge near him, attack. When he's in the air, focus on your upward attacks - Ariels, tilts and smashes.
It's difficult to hit him with precise attacks like f-air and b-air. After ^B, though, he leaves himself open. Keep him off the ground, since that's where he's strongest. Keep in mind that he's quick. Zelda's generally do better in defense against quicker players. Incorporating Sheik into your Pika-battles should help you out quite a bit. She can't deflect his moves, but she is just as quick, if not quicker, than Pikachu.
That's pretty much all I have to say.