The move tutor in the Colosseum Marketplace teaches Volt Tackle to Pikachu, so teach it before you evolve it for $35,000 or 50 BP. You reach the Battle Colosseum after the Arc Badge.
A Pichu egg gets Volt Tackle when either parent holds a Light Ball, so you need no father that knows it. Breed any Pikachu or Raichu with one held.
How breeding works here
Leave a male and a female that share an egg group at the Lagoona Day Care. The egg hatches as the mother's first stage.
Either parent passes on an egg move it knows, however it learned it.
A Pokémon that is male only or genderless breeds with Ditto. Ditto passes on nothing.
Pokémon from codes, the roulette, Looao or tokens cannot breed.
Some babies need an incense on the mother (or on the parent paired with Ditto): Munchlax needs a Full Incense, Azurill a Sea Incense.