as long as it's ingame, one pokemon is enough. I think one pokemon taking on all of the ingame part of the game is easier than having a team. Then you focused all those trainer experience onto one Pokemon to make yourself super high level. The only problem is restoring PP for the elite 4 if you use one Pokemon.
As a Blaziken, use this moveset (ingame only)
-flamethrower
-rest
-rock slide/mirror move
-brick break
there's no better way but to level up your Pokemon. Be at least lvl 55 for all of your Pokemon (60 recommanded). Dusclops and Absol really waste your PP well so be sure to rid them first. The only problem i went through when i'm facing the elite 4 is out of PP, so you may want to bring a few items that can restore PP.