You don't. It's a well-known myth. More info at Bulbapedia:
http://bulbapedia.bulbagarden.net/wiki/Catch_rate
with a pokeball
Team Magma Hideout
use gamesharkDCEEEC34AFEADC26 masterball is pc
its in a room in magma's headquarters. for more Pokemon info go to serebii.net
If the team magma hide out near lilycove city has already closed you might as well just start over. In the Team Magma hide out there should be a room with four pokeball beside each other, 3 of them will be voltorbs and 1 will be THE ONLY MASTERBALL YOU'LL GET. If you dont have a masterball, the catching rate for Groudon is 1%...so make sure you get that masterball! The first time you throw the masterball at groudon it'll work so dont worry about wasting it on groudon
You get it from a guy at the top of a big building in the first town you get to
inside team aqua's base near lilycove city
theres one materball in team magma/aqua hideout
you have to beat the game and talk to the professor and he will give you one
Each Pokemon game has one Masterball..
on the legendary Pokemon that you encounter on the game thx for reading x --:)--x
I recommend the master ball