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I finished white in 10 hours in a speedrun i did
If you really want to finish it as quick as possible just trade a strong underleveled pokemon
If you want to use pokemon caught in the game, i reccomend to dump your starter they're shit
In B/W get Lillipup, Sawk/Throh, Petilil, Sandile, Darumaka or Reuniclus, they are strong pokemon caught early on that dont take a shitload of time to evolve, also, get archeops that shit is OP
In B2/W2 you can get strong pokemon like Riolu, Magnemite, Elekid, Magby and Eevee early in the game too.
I would say 3 pokemon is enough to defeat the E4 if they have the proper moves, oh an use TMs.