There are 2 ways... Easy or Hard...
Easy
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Battle Pokemon with weakest and don't let your Pokemon GO 10 levels apart of the weakest one.
Hard
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Same as above exept you EV IV train them... Your Pokemon would be like Pokemon on starieds
It helps you recruit Pokemon easier if you have more Friend Areas. Also, it helps if your Pokemon have a high enough level to make a good impression on wild Pokemon.
The Nest Ball makes it easier to catch Pokemon of a low level.
a easier way of leveling up is below! Pokemon level 1-30 find a level 50 Pokemon Pokemon level 31-100 find a level 100 Pokemon Pokemon level 101-200 fight a level 200 Pokemon Pokemon level 201-300 fight a level 300 Pokemon and so on... WARNING YOU MUST DO THIS ON VERY EASY!!
The best way is to buy a load of rare candy its a lot easier
Dialga is easier to catch Palkia is a bit tougher to catch but Pokemon level 75 will do the trick.
You would have to level up to at least level 25.It will be more if it is rock/ground.It would be easier to raise a starly, honchkrow, or zubat.
you would want it to be over the level of the Pokemon league's Pokemon by ten more levels and try to study their Pokemon and types so it would be easier. the level would be 70-80.
If you already beat the game, just battling the Elite Four is a great way if you can take the repetition!
The best legit way to quickly level up your Pokemon is by using a lucky egg, which doubles a Pokemon's exp after a battle. Or just use rare candies derived from an action replay if you are okay with cheating.
Everyone knows that pokemon only go up to level 100. Ditto evolves at level 20. Hope i made it easier for you to train your ditto!
it would be easier if you are higher than a level 50 cuz there is about 6 Pokemon that are about up to lv 50. the highest level is 58. so train higher than that:) good luck
Trade it for another Pokemon if you trade it for the same one you have using GTS in Goldenrod using GTS you will get the same Pokemon. But do know that traded Pokemon grow faster and get more exp points at the end of each battle. They are easier to raise than Pokemon you catch.