You should swap your party Pokemon with a friend and get them to level them up so it gets double the exp point. Also you could give it an exp share and take it to the Pokemon league, if it is too weak to fight then it doesnt have too but it still gets alot of EXP the average Pokemon will grow to level 32 if you take it through the Pokemon league with an EXP Share (I take it through with a level 100 Rayquaza so this may vary for other Pokemon).
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The easiest way to level up is to feed Pokemon Rare candies.
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.
It's mewtwo I have one he/her never dies but you have to level him up to your highest level to be the best I have a level 97 Pokemon that's way stronger than mewtwo he's level 80. It depends on level. If you are talking about lv100 it is mew as mew can learn any attack
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.
there is no way, all you have to do is the slow boring way. sorry