Here goes:
This matchup is indeed very hard for Pichu. Definitely one of his hardest matchups.
You'll want to avoid his Aerial attacks because if his huge priority. Remember: ANY of his aerials own ALL of your aerials in both range an priority, except your U-air, which will trade hits. That's why you create openings, which is what mindgames are used for. Imagine a Luigi jumping up and planning to attack you from the air.
You don't: Try to attack him when he can throw out any aerial anytime (that nair really does come out quick)
You do: Trick him into attacking.
You either: Do a quick empty shorthop, making him think you´ll attack, so he´ll attack too.
You evade the hit and attack him in his lag.
Or you: Do nothing/dashdance a little for confusion. When he's coming down with his aerial to hit you, dash away to evade and dash back to counterattack/grab. This should work, considering the very little momentum Luigi has while moving forward in the air.
OR you: Think up your own mindgame/manoeuvre. This is only 1 of thousands situation a smash match can get you into.
His Wavedash is annoying. His smashes beat the crap out of you and kill you at barely 85 percent (rough estimate). Avoid this, and punish. It's hard, I know but there's really nothing else you can do against it besides avoiding and punishing. Don't try to shieldgrab his smashes. Jumping out of shield with an aerial is often useless too, as he moves too far out of range to get hit most of the time.
Edgeguarding his recovery is hell. Just try it, it's pure evil. It's one of those recoveries everyone can edgeguard pretty easily, except for Pichu ._.
You might be able to combo him, but not with U-air. Nair works like usual.
The conclusion is really to play smart, it's VERY hard to punish a Luigi who spaces good for making mistakes, especially when you're Pichu. . It's quite impossible to beat a great Luigi player like uhm....Drephen with Pichu. In a tourney match, I'd advice you to pick Pikachu. A lot better choice against a Luigi.
I probably forgot some stuff, but whatever. Hope it helped ^_~