Since you're familiar with your own character, dittos always revolve around one thing: what does your opponent want? It's extremely important to observe your opponent's habits and find ways to counter. Pikachu players are often aggressive, and have a tendency to QA into you or land behind you with a Fair, both of which can be punished with grabs or Utilt. You're looking to win with a better understanding of what is and isn't punishable, and you'll spend a lot of time in aerial neutral with safe SH / FH Fair fades. These matches are just arduous and long, but think about it like the Fox and MetaKnight match ups: in a speedy-versus-speedy matchup, it's all about who can space the neutral and punish mistakes.