I wouldn't say I'm very good at Ultimate yet but I play both Palutena and Pichu. Here are my quick thoughts about this, but take it with a pinch of salt as this is from personal experience, where I am not by any stretch a master of either character, and there are other players who would be able to give a better idea about this matchup. Also, I don't really play this game all that completely so there's that too.
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Pichu is quick so you should actually be able to avoid EF and punish Palutena for using it, especially if they spam it. Autoreticle is also a poor choice against quick characters, especially one as small as Pichu. Pichu's weight is a scary issue as Palutena can send you flying with a well placed FSmash, USmash, Ftilt or EF. TJolt and your speed can help interrupt her if she attempts to zone you a bit.
If you stick to her she won't be able to use her Specials much, and her tilts become a gamble as they have quite a bit of startup so you can punish them. Smashing you basically become an impossibility so long as you are within range of her. Pichu's size and weight are actually a boon when it comes to Palutena's Nair, it helps reduce the number of hits you take from it at times (especially if you aren't jumping) as well as making it harder for Palutena to combo Nair into itself. At close range Palutena will probably look to Jab and Throw, so be ready for that. So long as you smother her and don't let her play the mid-range game for too long you should be able to do well. Keep her planted on the ground as much as possible and she'll have to play there, where she is weaker as she much prefers the air.
Also, remember, her dash attack has invincibility in front of her on some of its frames.
If you learn to read Palutena's DSmash it becomes incredibly easy to punish and one of her worst attacks, most Palutena's actually avoid using it as much as possible.
Finally, while her counter is actually "good" (it's still a counter) in this game compared to SSB4 it all comes down to if you can beat her to it or if you can mindgame her into using it and then punishing her for it.
I can see this matchup as being difficult, as Palutena specializes in punishing and is quite fast, but in my opinion, Pichu should actually have the advantage so long as you don't let her get a foothold on how she wants to play and you keep her guessing and use your speed to keep her from using her strongest tools.
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I apologize if anything is incorrect or doesn't make sense here. I am still learning a lot, and am definitely not an incredible Smash player. This is just my thoughts from where I am sitting as a semi-competitive/semi-casual player and I expect someone to correct me on something lol.
