Simple. To level up your Pokemon quickly, all you have to do is battle trainers. This is the way I do it. You could also put them in the daycare. You can battle wild Pokemon, but I don't think this is very efficient, unless you've just started the game. But I'd only battle trainers with the same level Pokemon as yours. Edit:(NewPerson)I found out a different way. You can get an EXP share and then give it to the Pokemon you wanna level up, then just eat everything with your main. I'm going to test it, and I'll edit this as soon as I can.
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Ok if you want to get a Pokemon stronger you need to have a great bond with it and it needs a great bond with you. You can find a place to give them a good place in the cycle. The cycle is(fire vs snow. fire wins)its like simple math. Like snowpoint Pokemon vs Infernape. I think you know what will wipe out most the snow type. And before you know it BAM level 50-65.
Training a Pokemon that was traded to you will give that Pokemon boosted exp points also holding a lucky egg will also give double exp points. Seven island is the best training spot.
Use the versus seeker which you get from the girl in the Vermillion Pokemon Center. it lets you battle people you have already battled. Gets you a lot of experience and also money
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
The fastest possible way is to train a Pokemon someone else gave to you and have it hold lucky egg.
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.
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.