The fastest possible way is to train a Pokemon someone else gave to you and have it hold lucky egg.
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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
A good place train is the route next to Undella town. You can put exp. share on a weaker Pokemon let a stronger Pokemon do all the work and you will level up quickly because the Pokemon there are usually about level 50-60. Another way is by using the daycare to level up fast by just using your bike or running to level up.
take it to a Pokemon daycare after you beat bugsy and learn the move cut or get alot of rare candies or battle tons of trainers
the easiest way to level up your Pokemon to put it in daycare Pokemon will level up soon.Trade the Pokemon to friend and have it hold a lucky egg it will level up quickly.