Depends which version of Pikachu we're using as its power is difficult to judge when talking about different continuities, in-game stats vs Pokedex entries, etc. For a more composite version of Pikachu based on Pokedex entries and the peak feats of Ash's Pikachu, it'd be Large Mountain level from Ash's Pikachu being able to easily fight a lot of powerful Stage 2 Pokemon such as Tyranitar or Abomasnow as well as Z-Move/Gigantamax Pikachu being stronger than a lot of users of the move Earthquake. Speed-wise, a composite Pikachu would get Relativistic+ speed via scaling from
Gyarados and Golem dodging a Solar Beam. Using all that, It'd at least take Kami to take a Composite Pikachu down, or possibly BoZ Piccolo as the massive speed advantage it'd have and its stamina may be enough for it to solo Part 1.
Including its Blue Orb self from the anime, it gets boosted up to Multi-Continent level from being able to fight Kyogre, assuring it solos Part 1 but still loses to the Piccolo that destroyed the moon.