In my opinion, I always goto the elite four (aftergame) and replay them. You can also goto the east side of Unova and fight wild pokemon.
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There really isn't a quick way to get to level 100 fast without cheating. The best way would be to re-battle gym leaders and the elite four, along with wild pokemon. Sorry, you'll just have to be patient.
There is not really a easiest pokemon to evolve...but bug type pokemon evolve really fast
Easy really there are (or were) certain accounts people made for the exact purpose to level your Pokemon. First you would find tab that said "battle computer playor" enter thbase name and type of Pokemon you would like to level up. It would be like "Zedjarock", or "Zedjalightning", etc. some more effective than other. >_<
Defeating Pokemon that are a higher level that it, or Rare Candies.
there is no way, all you have to do is the slow boring way. sorry