I can come with some insight, since I play Pikachu pretty frequently.
Crossup Nairs is like Stride say their "BnB", either CC those at lower % or try WD over and sneak under an Usmash, or just outspace and poke with ftilt,downsmash etc.
Pikachu grab. Atleast the one I'm playing, fish for grabs at the edge, cause Luigi's poor air speed, you can get easly gimped at the edge by the tail. Don't play too him, play safe. If you do get grabbed, try mix up your recovery back with airdodge, missile.
If you get grabs mid-high%, down throw turnaround Fair/Dair. At lower %, Uthrow too any ariel, maybe you even can chain uair->something.
Utilt is great in this matchup in 20-40% range, can even get 2-3 in a row, and is still a great setup for ariel.
At ledge, they often try to get you off with thunderjolt if they recover high, you just have too give them edge if they are about too hit you. When they start their quick attack, and you are hanging at the edge, it's a good chance they go for the stage. Walkup->UpB or even ledgehop Dair where you think their quick attack is comeing is nice.