To defeat the twins, you need a powerful water-type and a Pokemon that can use dark moves (preferably dark time). I recomend having an absol in your party that can use the move bite and has balckglasses equiped.
YES, YOU DUMMY!(after you defeat the Pokemon League and get the Ruby/Sapphire.)
Get the Goundon
have a strong fire Pokemon or fire move
yes swarpert is a greatest Pokemon to defeat elite four
by fighting
defeat the pokemon league
You can defeat trainers and sell items.
you need to defeat the elite four on both games
I'm going to assume you mean "Pokemon Ruby" in your question. Anyway, to trade between Ruby and XD, you must beat Pokemon XD and defeat the champion in Ruby.
get a fire type
We can get other regions starter Pokemon if we defeat the battle tower
Defeat Flannery of the Lavaridge Gym, and then return to Petalburg City to defeat Norman of the Petalburg Gym.