_a very fast Pokémon level 40 or more that knows Spider Web(like Ariados),Mean Look(as Golbat and Crobat),or Block.Is better that you give Rapid Claw at this Pokémon;
_A Pokémon level 40 or more that knows a paralyzing move (like Thunder Wave or Stun Spore);
_A Pokémon level 40+ that knows False Swipe (like Sneasel,Weavile,Scyther and Scizor);
_Max Repellents,Ultraballs,Timerballs,Darkballs,and,ifyou want,Weightballs.
AN ADVICE:do this part in the evening,you will have more catch rating and luck with the Darkballs!!
1)Go to a city,I suggest New Bark Town,because has only one near Route and is more simple in this way to run into the fugitive Legendaries;
2)Continue to go to and fro the city and the near Routes,until that the Legendaries arrive in your same Route;
2)Spray the Max Repellent,and go in the grass with the Pokémon with trapping move at first place in your party.Pokémon MUST to have 105+ in Speed;
3)Use IMMEDIATELY the trapping Move.Therefore,replace it with the Pokémon with Paralyzing Move;
4)Use IMMEDIATELY the paralyzing Move.So,replace it with the Pokémon with False Swipe;
5)Continue to Use False Swipe.When the Legendaries will had only 1 HP,throw it all the Ball string you have!
6)Hope with all your heart that the capture succeeds!
I hope to have helped you!Write here if these are good advises!