Pika vs Kirby:
Pika needs to aim for an edgeguard chance, because Kirby is incredibly vulnerable when he is recovering. U-Air chains are great for forcing them off. Again, other combos aren't discouraged, but finding a chance to edgeguard is definitely your first priority.
Small Advantage for Pika on all stages.
Stages:
Good: Dreamland, Congo
Okay: Hyrule, Peach's
Bad: None
Dreamland is generally best, as it is the best for gimping, though more cautious Pikachus can choose to go to another stage, as Kirby is also excellent at gimping. Kirby also has very efficient air camping on Dreamland, so you have to be proactive about preventing this.
Congo is beneficial to Pika, as Kirby's recovery is still very easy to edgeguard here and Pika is overall faster than Kirby, so he is a bit harder to edgeguard with all the options available to him.
Pikachu doesn't have any particularly dreadful stages on this matchup, as any stage strengths Kirby has aren't really enough to negate Pika's natural advantage.
Peach's Castle may also be one you would wish to avoid. Kirby is just very effective on Peach's, with plenty of platform options and easy edgeguards with D-Smash/D-Tilt on the moving platform. He isn't bothered a great deal by the bumper or side triangles, given he doesn't often KO off the top, and can garner gimp KOs easily. While Pikachu is hardly disadvantaged by the stage, any stage which offers benefits to your opponent isn't generally one you want to pick. Remember Pika has some pretty good stuff there too.
Approach
Do not rush headlong into Kirby with random dash-grabs or U-Smashes, as he is the best punisher in the game at low percents (and probably high percents as well). You hear that Kirby can sit there, spam U-Tilt mindlessly and still do okay. You have to either approach between U-Tilts or approach from the front. Since Kirby can just turn around, the first option must be practised. F-Smash clanks with U-Tilt, by the way, so don't get any silly ideas.
U-Air beats Kirby's options from basically every angle so long as it is used properly. This include U-Tilt, though keep in mind that U-Tilt is fast enough to be counted as a wall/constant hitbox, while U-Air needs to be timed.
B-Air is also an option, and will generally at least trade hits with Kirby's moves (it may outright beat them), but U-Air is generally better because it leads into other moves at a wider range of percentages.
It is safe to hit a Kirby in the air from above with (fastfall) U-Air/D-Air/N-Air because he has really, really poor hitboxes reaching above him. The only one of any note at all is Up-B, which no Kirby will be able to consistently use against a Pika without huge risk. It's really narrow anyway.
Combos
U-Air chains work at a lower percentage than they do on heavies/fastfallers. If you U-air as you near the ground, it leads into dash grabs, pivot grabs, U-Smash/pivot U-Tilt or even a pivot D-Smash (use to get edgeguarding opportunities, slightly stronger than F-Throw).
Any lead-ins to U-Tilt should be used, and followed up with an U-Air combo, B-Air or double jump Thunder (certain percentages only, and risky anyway). U-Air > dash pivot U-Tilt, F-Air > U-Tilt, aerial > U-Tilt, throw in the **** tent > U-Tilt are all possible options. Weak U-Smash can be substituted for U-Tilt, as it is weaker but leads into the same things (except Thunder). This should generally apply only if you miss the actual U-Smash.
Defense
The big threat here is U-Tilt, as getting hit instantly pushes your percentage up without reservation. Luckily, Pika can't be juggled extensively only with U-Tilt, but you still have to be careful.
Pika's own U-Tilt can often beat out Kirby's D-Air, but it has to be timed right. Immediately jump to the offensive if you land an U-Tilt.
Be very, very careful about being baited. Kirby's multiple jumps allow him to air camp you until you do an U-Tilt or jump up at him, which will allow him to drop in and D-Air/N-Air/something you. Wait it out until he lands, but if he is on a platform above you, be careful of jumping into D-Smash, U-Tilt or platform dropped aerials.
Even head-on, Kirby's N-Air and B-Air are substantial, are can even beat Pika's U-Air from the right angles. Hit them from below or above if possible.
Recovery
Kirby has big aerials that stay out for ages, and his F-Smash, D-Tilt and D-Smash on-stage are quite dangerous. That said, he is sort of slow and puffy, so as soon as you get past a Kirby who comes out to aerial you, you can retreat into the stage. If he edgehogs you, Kirby has no particularly quick way to get in-stage, so you should be okay. The focus is on avoiding those aerials.
Edgeguarding
If Kirby is low, wait and F-Smash or fall with an N-Air. Kirby won't recover low if he can help it, as these two options (or just one of the two) will shut him out very quickly. N-Air is recommended at low percents to stop Kirbies hugging the stage. This stops significant reverse ledge DI tricks. Otherwise, use F-Smash, because if he isn't hugging the stage, he can move very slightly and cause you to miss N-Air.
Most Kirbies recover high, because this is safer. If he is moderately high, you can often B-Air him. Be careful, as most Kirbies will use a jump and F-Air to maximise forward momentum. B-Air will beat it easily if spaced properly. U-Air beats it even without spacing, but it isn't as strong. If you miss your aerial, you might be able to try again with N-Air/U-Air. If Kirby somehow dodges you and gets past, aim to grab the ledge with Agility and refresh your edgehog if needed (if you're on the left edge you can use Agility again).
If Kirby is really, really high (I can't actually think of how this would happen aside from them going so far they brush the blast lines), then you can either let him return or risk using a Thunder. Thunder is incredibly punishable, especially if you jump off to use it, but is basically a KO if you land it.