keep trying to beat them
you have to defeat elite four
You have to defeat the elite four then you can catch the legendary
yes swarpert is a greatest Pokemon to defeat elite four
You get Reshiram at N's castle after you defeat the elite four.
defeat the elite four
The Elite Four in Pokemon Black/White is quite simple to beat (in comparison to the other games), but what you have to do next is defeat N, the the leader of the Seven Sages, whatever his name was.
After you defeat the elite four on Pokemon sapphire you complete the pokedex and make all your Pokemon level 100.
You can catch Reshiram once you battle and defeat the Elite Four. The story line will basically guide you to Reshiram after defeating the Elite Four.
you defeat the elite four by training a particular Pokemon and getting it to a high level(e.g. lv64) you also give it the best four moves that you have. try to give it moves that have a lot of pp though because you need to defeat the elite four who have a lot of Pokemon!
After you defeat the Elite Four.
Wailord
Defeat the Champion in the Elite Four.