This manga doesn't pull any punches in regards to the sheer power of the Pokémon and the gruesome consequences of using them.
I'd like to think that the explanation to Pikachu's win is that while Red's other Pokémon were fighting, it was resting and accumulating electricity. The moment it felt threatened by Onix, it released all that electricity in a single attack that shocked Onix to pieces. Still an a**pull but hey, we've seen worse in the anime.
By the way, badges boosting Pokémon stats is a thing in the games up until Gen 3.