In Pokemon Leafgreen or Firered, you should try and catch the one last legendary Pokemon with your master ball if you saved it. (It depends on your starter Pokemon on what legendary you can get.)
DANCE
Yes.
Not a thing
go to Viridian City, go west, and challenge the Elite Four
You can only catch it after beating the elite four however you can get it from a friend before defeating the elite four.
Go to the battle tower
Yes you can
It is part of the storyline yes, after defeating the elite four, a lot more things become available.
we can get it by defeating the johto elite four
You can go on to defeat the Elite Four.
Sky pillar which only appears after defeating the elite four.
After defeating the Elite Four once, their Pokemon are much stronger than before, and with new Pokemon. There is no new Elite Four to battle, though.