There are many ways to level up your Pokemon in Leaf Green version.
Use the Vs. Seeker:
Be sure to use this tool often. This is a key way to gain experience by battling trainers over and over again. For instance, when I was training my level 52 Raichu for the Pokemon League, I went to Bond Bridge and surfed to the trainer near the rocks. She has a Kingler at level 38, and when I beat her, my Raichu gained about 1,600 exp. points. I used the Vs. Seeker, and was able to fight her over and over again, increasing 1 level every 3 minutes. (Approximately.)
Use Exp. Share:
This item is extremely helpful for raising a Pokemon. The experience points that your Pokemon receive are doubled when you give this item to a Pokemon to hold. I recommend combining this item with the Vs. Seeker to get maximum experience points. To acquire the Exp. Share,
Helpful note: The Exp. Share will also allow your Pokemon that does not participate in battle to receive half the experience points that your fighting Pokemon receives.
To acquire these items:
Vs. Seeker: Where do you get the vs seeker in Pokemon fire red
Exp. Share: Where do you get the Exp Share in Pokemon Leafgreen
squrtle
Raise to level 20.
Mewtwo? Plus you cannot get Mew in Leafgreen or Firered.
doggy biscit
To get Seel in Pokemon Emerald you must trade for one from Pokemon FireRed or LeafGreen. Seel can easily be found in the Seafoam Islands on Pokemon FireRed and LeafGreen.
Well you can Easily get moltres zapdos and articuno
It depends on your lv probably the toughtest place you were in.
train it at the Pokemon league
Beat the Pokemon league and fill the pokedex. Raise a team, and challenge all of your friends to be the best.
Use vs seeker and train at six/seven islands, maybe......
At the Elite Four or battle NPCs with Pokemon the same level as your one.
start game with charmender .catch a pidgey,pikachu,nidoran♀,geodude. raise this pokemons and after you have raise enough you should try to catch legendary Pokemon