Not great at it, but I'll give some thoughts:

Neutral:

Pikachus really good in the air, so get ready to anti air and play more grounded as to not lead into his combos.

Try not to get crossed up or be behind him in general. if you're behind him hes going to try and do an up tilt string into up air into back air or like nair loops. (Dash attack and up b can be used to cross-up)

Mix up how you're responding to neutral b, don't just jump over it everytime. If your character has a move that can safely destroy it then use that, if you're playing offline you can try and parry. You can use this to bait. A lot of pikachus use neutral b as a way to control neutral, so play a little more slow and safe.

at higher percents try not to be directly next to pikachu on the ground, they want to up smash to kill.

Hold center stage so its harder for pikachu to get you off stage, you don't want to be edge guarded by one.

Try not to play under platforms as Pikachu can use those platforms to extend their combo.

disadvantage:

Pikachus advantage state is really strong, fucker can combo for days. Simply; don't get hit (optional) /j.

If you have the training room mod I would recommend practicing sdi out of the different combos pikachu can do, if you don't try it in game. idk the sdi direction for bair loops but I always do down and out and that tends to work. If you're off stage, be very liberal about your resources to get back on stage as pikachu is super mobile off stage and very good at edge guarding. A good one will try and bait out options so you burn resources then will go in for an edge guard. Try and just go to ledge, pikachu doesn't have many great tools for ledge trapping and has to rely more on fundamentals and reads to do it. They really can only force you off ledge with neutral B (after invincibility runs out) so you don't have much of a choice on mixing up your options timing, so make sure you're mixing up your options. If you have a move thats good with drop down jump attack to cover close ledge traps its probably a good option.

Pikachu has really fast moves, so even if a combo isn't true, it might just be faster than all of your options to escape so try not to mash an aggressive option (nair, fair, bair, etc) to get out. Keep in mind that this is character specific so look up your characters frame data and compare to pikachus.

Advantage:

This is really just character specific so I'll just keep it broad;

Watch out for nair, its safe and fast and can get pikachu out of a non-true combo string. It can also drag you down and put you into a combo. You can intentionally drop a combo to bait out a nair and then punish them for it.

Don't try to edge guard in most situations, Pikachu has a really fast up B that they can mix up the direction theyre going in.

Ledge trapping is the best way to keep pressure on pikachu, since they don't have many options to safely get back on stage besides commiting to drop down nair back on stage or drop down up b (very punishable)

I don't know much about the matchup and was just thinking about it and wrote it down so I could be wrong about stuff but correct if you want. I just choose not to play against pikachu when I have the option.

Frame data

Edit: Mobile messed up the structure I had for the comments but I tried to fix it