you must catch 60 Pokemon
No You don't.
get the ruby and the Sapphire gems from 4and 6 islands . the go to ceroolien cave.you can find mew2 in there.
first you bet the elite 4 and then go to ciolol and then you need to find the gems
you find every single Pokemon and then you go to her and then you ask twice and she will gve you three moon stones, five fire, and two sun.
With the dotted holes in leaf green, that is where you get one of the gems to get inside the Rocket Warehouse. Just have a Pokemon that knows cut, go up to it, then press START, POKEMON, ???, CUT. It's that easy!
You get suicune,raikou,or entei depending on your starter pokemon (charmander = suicune squirtle raikou and bulbasaur entei) and you can get mewtwo in cerulean cave
you go to the elite four which is to the left of the place you got your 8th badge. you need to cut the tree then go through a cave to get there. it'd be a good idea to have Pokemon with levels over 60!
no to trade between leaf green and sapphire or ruby u must beat the Pokemon league then go to the seven islands and find the ruby and the sapphire gems to be able to trade
the underground
Nope, there's only one. In case you didn't know this, after you've done the stuff for celio on the sevii islands (getting the gems), then you can go back and re-challenge the elite four. They have different Pokemon and are a bit stronger than before.
Gems weren't made yet but they didn't hide the bar
I think you get gems by playing Sableye's Gem Journey