I got some time and decided to practice this match up then share my knowledge on what I learned.
Camping:
Pikachu will thunder jolt you. Thats his only projectile lol. The idea for Pikachu is that he's trying to look for a way to get in. This means it is quite critical that you do not allow the thunder jolt's to hit you. Now power shielding is sought to be the best method to prevent this, but I found other methods. The thing with power shielding is that your waiting until the thunder jolt hits you. If you are far away, this is not something you want to do. The thunder jolt is basically Pikachu's body guard and as you let it reach your body, its essentially letting Pikachu reach your body which is not good. Its about knowing when to power shield (which I'll cover a little later). Keep in mind the thunder jolt shield pokes.
Olimar definitely does out camp Pikachu. Yes there is the fact that the frequency of our pikmin toss is much higher than that of Pikachu's thunder jolt. But Pikachu uses aerial thunder jolt's. This is to allow Pikachu to stay out of range from Olimar's pikmin toss. Its also to allow the Pikachu to plan his movements so that he can get 2 thunder jolt's running across the ground near each other, thus increasing his control on the ground. Do not attempt to double jump and hit Pikachu with your pikmin. Its literally a trap. Pikachu will quick attack over to the ground your above, and you'll be in your worst position since your above Pikachu.
Heres what you should be doing. Remain grounded. And now this is how you beat Pikachu's camping. With a grounded pikmin toss. The idea is to have the pikmin take the damage for you. Its a little hard for me to determine which pikmin is actually the best for this scenario because they each have there pro and cons, so I will just go through all of them.
Red/Blue: They have the same trajectory. Nothing to special about them. They may be the worst to take on the thunder jolt seeing as to how they take damage from it, but at least they stop the thunder jolt, so they're not useless. But I wouldn't be surprised if red was the most useless pikmin in this match up. At least the blue's can kill from throws.
Yellow: Obviously they can absorb the electrical attacks. But it doesn't mean it's the best against thunder jolt. I'll explain why shortly. What happens with this pikmin is that it will go through the thunder jolt and still continue its normal path. Obviously, but, the problem is the trajectory. The arc that the yellow follows is quite impractical. The red/blue pikmin follow a better arc. So basically, in terms of defense against the thunder jolt, you need to use yellows with proper spacing. From far away, it is ok since it will block the thunder jolt as the pikmin is landing, but from up close, Fsmash the thunder jolt. The Fsmash can also hit Pikachu on the other side. Its an effective means of destroying Pikachu's approach.
White: So this pikmin has range as its largest advantage. It will be able to cross a large area and stop thunder jolt's from afar. However they can't tank if they're life depended on it. It may be a better means to save them in terms of preserving them, or attempt to hit Pikachu with them due to the fact that since they travel far, with effective use, you can get away with try to gain a higher angle by jumping off the ground. Then you could land and shield the thunder jolt. Or as you're landing, you can Nair the thunder jolt. The Nair does beat it, every time. The other reason why you might not want to try and beat thunder jolt with the whites is because when other pikmin are standing up from a toss, they still have a hurt box and can still tank the damage from the thunder jolt.
Purple: These have proven to be exceptionally useful. They tank so hard, that they actually do not take damage from the thunder jolt. I beats the thunder jolt with its hit box and it still maintains it's hit box to hit anything behind it. Unfortunately they to not travel far.
Some other things to think about. If Pikachu set up 2 thunder jolts. Yellows Fsmash and purple toss are the best way's to beat it. Otherwise throw one pikmin to take out one of them, and power shield the other. If you are close to an aerial Pikachu, you should power shield the thunder jolt since it will still be air born. At that point you can attempt to punish Pikachu with an aerial. This is usually safe. I also tried short hopping over the thunder jolt and that also proved to be impractical. we will only avoid it if the thunder jolt is at its lowest point, other wise it will hit. Short hop camping isn't a great idea. Utilt and Jab also clash with thunder jolt. They are both impractical. I wouldn't be surprised if Dtilt, dash attack, Ftilt also clashed. They're all dumb for this purpose.
Pikachu's Quick Attack:
Its quite a versatile move. When canceled, it can go into any aerial, so careful for that. This move however is not much of a problem at all. Olimar's Fsmash goes through it with any pikmin. Olimar's Nair and Utilt also go through it. The Utilt can be used as a good setup for Usmash, or a landing Nair also works. To be honest, the viability of the quick attack as an approach is completely ruined due to this. the best Pikachu can do is use quick attack for movement. It won't work as an offensive against us unless we are above Pikachu, and he quick attacks under us then proceeds to Uair juggle.
Aerials:
Pikachu's Fair is one of his most important setups. His Dair can be used to shield pokes. when both are used on our shield, they can be spaced so that we can't UsmashOoS. The Fair can be shield grabbed, but not the Dair (unless they eff they're vertical spacing off a short hop/use it while falling and get the landing lag). Yellow pikmin outright beat this move. The hit box is the electricity. Therefore any pikmin attach that uses a yellow will beat it. Purples toss will also beat these moves, but not anything else with a purple. Everything else will get out prioritized.
Pikachu's Bair is lulzy
Pikachu's Uair is mainly going to be used to juggle you. If your shield is small enough and your on a platform, It will shield poke you. Theres not much more this move will be used for offensively. It can knock off pikmin really well, and it has good range so it can out space Olimar's Dair (not sure with yellow though).
Pikachu's Nair is going to be used rather strategically as well. Its knocks off pikmin better than any aerial, but that will stale the move. That being said, be ready for Pikachu's to OAC into you and Nair. It will be used as a kill attempt, or at least to get you off stage for gimps. Its probably the most punishable off all of Pikachu's aerials because its best hit box is the initial one. The lingering hit box can be beating by about anything. Any attack, any aerial."]
Edge guarding:
Pikachu will use 2 moves mostly to prevent you from coming on stage. Thunder and Nair. You can whistle the thunder, if you want to be able to act quicker after avoiding it, or you can air dodge through it. I've attempted to punish a thundering Pikachu be Uairing him as I recover (even while he is getting hit by the thunder), and it doesn't work. Perhaps with a yellow tho, but for any other pikmin, you will just hit lag the thunder, and end up getting hit by the sweet spot of the thunder. Due to Pikachu's good recovery, the will come out at us and attempt to Nair sometimes. It doesn't even have to sweet spot, as long as it hits us away, it will hinder our recovery. It can be beat or whistled through however, and once you pass Pikachu, he'd only be able to quick attack back on stage, so its fairly safe for you to get back to the edge once you pass him. As for Pikachu's Jab, it does stop our getup attack/normal getup, but its bad for Pikachu to try this. we can drop down, double jump Uair him. He can also Dair as an edge guard which stops our jump on stage from the ledge option. So those are some things to think about.
Other stuff:
Pikachu's jab close range is guaranteed damage on our shield. Get out of it asap. It can trip you and lead to a good setup for Pikachu. Pikachu can also jab over the edge of the stage if your not going for the ledge when recovering. I you are caught in this, SDI on stage. Its not safe to SDI away and potentially get gimped. Pikachu can Usmash after an Fthrow, but has no chain grab on us (Fthrow/Dthrow) not sure what his Dthrow follow ups are. If you are above Pikachu after getting hit from a Utilt/Usmash, be wary of the thunder. Pikachu will sometimes thunder based on where your going to be considering the direction you are falling. He can also try and bait an air dodge out of you. Olimar has the option to whistle instead of air dodge since it will offer him some more freedom afterward. The air dodge is good to avoid damage. Also be careful for the hit lag that will be added on to Pikachu's moves. Because some pikmin may still have their hit boxes, it may help Pikachu to Fsmash or Dair while they are around since the hit box will lag and mess up your timing if u attempt to dodge these moves for example. When we down throw Pikachu, its hard to follow up after like 10 percent because of his aerial mobility. He has a enough air speed to make it near impossible do re grab him after a Dthrow. The combos work best at 0%.
Stages:
FD is the worst neutral. Platforms help Olimar, his projectile and overall move set works better with platforms. BF would a good neutral. Yoshi's and Lylat are also good. Lylat allows for more yellow's.
Halberd (low ceiling), Castle Siege (small stage 1, and statues stage 2) work as good CP's. Delfino and PS1 are also good. Olimar can use both stages effectively with the transformations. PS2 gives Pikachu some good advantages. The Ice stage makes his Fthrow more effective since he can follow up on it with more options. He might even have a chain grab (Fthrow) on the ice. not too sure. The air stage also helps Pikachu more than Olimar. Gives him more time off ground to camp, and he can QAC if he needs to get grounded. Frigate isn't horrible but its still bad for the general reasons.
Don't really know about Picto chat, but conceptually in that its a longer FD, and Pikachu can set up into the hazards well with his throws. His throws might also get some good follow ups with the walls. The ceiling is also higher, so that doesn't help us. Rainbow cruise isn't good either due to the amount of movement necessary on the stage. Pikachu has better mobility. The ship, and the top section aren't bad. Our combos can work at higher percents due to the momentum of the stage. Brinstar is bad. It ruins our ground game. Pikachu has a better aerial game so it has proven to be difficult to keep up.
I'd say FD is the worst neutral, BF is the best personally.
RC/PS2/Brinstar are the worst CP's with Halberd, CS, PS1 being the best personally.
