If you've already beaten them once, all you have to do is go back and battle them again. No tricks.
well just go to the place that the elite 4 is at and do what you did the first time
nowhere. You can just do it again to get experience.
The first member will not be there. Go to four island and find her. She will be back in the indigo plateu .And challenge then again!
you can go and catch mew and go find the old sea chart
nothing. you just beat it 2 times and that's all.
Finish your Pokedex And Beat The Elite Four once then the second time they have stronger Pokemon
you can beat the elite four for the second time and complete the national Dex
To get the second Elite Four, you must have already defeated the Elite Four the first time, and the second time you go to fight them, they will all be leveled up, and become the 'Second Elite Four'.
you battle the elite four in the indigo plateau
At first a gyrados, dragonair, dragonair, dragonite, aerodactyl. Then the second time a gyrados, kingdra, dragonite, dragonite, aerodactly.
Over 9000!
You mean the national pokédex? I heard that you have to beat the elite four the first time and have at least 60 pokémon registered in your pokédex, and Prof. Oak himself will give it to you.