Keep this up, and he can easily take down all legendaries plus Sephiroth, Kurt Zisa, the Phantom, the data replicas of Organization XIII (except Lexaeus... but then again...), the Lingering Will, Young Xehanort, the Vanitas Remnant and Julius in just one Thunderbolt, all at the same time!
And then explode by levelling up to Level 0. Anyway, a starter showdown would make more sense, but Pikachu is in everything. The eight-year-olds would melt down if Ash boxed Pikachu, losing ~90% of viewers, so why would they? Anyways, Ash will win because he always does. Even though Serena had zero battles.
If levels were in the anime, it would be Chespin: Level 15ish. Fenniken: Level 6. (Team Rocket EXP.) Pikachu: Level 100. But then Pikachu could then lose magically. Still, I'd place my bets on Ash winning.