Blaziken.He has to know double kick and rock smash for it.Or use a lvl.35 machoke or machamp that knows cross chop.
at least level50.otherwise you will not be able to defeat his slaking.
To beat the Pokemon Leaf Green version for Gameboy you have to get the Nation Dex and beat Raiku and Suicune.
After you beat leader Norman
If I'm correct you should beat team rocket.
Finish your Pokedex And Beat The Elite Four once then the second time they have stronger Pokemon
You beat Leader Norman, then someone gives you surf.
Keep using Marshtomp's Mud Shot, it will lower their speed, and then you should use Water Gun.
you fill your pokedex
I would say any fighting types.
after beating alder and seeing or catching all Pokemon
beat the Pokemon league.
mostly legendary and your first pokemon you started out with but if you dont have legendaries , use high level pokemon -50 through 100-