Go to the area around Lilycove City, there's a whole bunch of lv. 27 Pokemon, but they're almost all ghosts. You're Pokemon won't faint that way.
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If if its a lower level like 10-20 you should probably go to the cave on the mountain where you fight team aqua.
No if you want one you will need to trade a dratini from firered or leafgreen to sapphire then train it to level 55 OR purify the shadow dragonite in Pokemon xd gale of darkness then trade it to sapphire.
In route 1 thas the best way to train pokemon if your pokemon lvl `10
if you mean what Pokemon are easy to train, the easiest Pokemon to level up and stuff is your starter Pokemon. it's the same with Ruby and Sapphire, the start Pokemon always seems to level up quicker than caught Pokemon (at the beginning anyway) hope that helped :3 *Shi ~
route 205 for Pokemon diamond, pearl and platinum. the trainers have Pokemon from level 10 to level 17