I find dusk balls much more helpful.
you can catchas them with timer balls whene they are weak
In order to catch your legendary Pokemon in Pokemon Sapphire, save your master ball For Rayquaza...you can catch KYogre/Groudon/both with either an Ultra Ball or Timer Ball.
It becomes more effective the longer the battle goes on. This is why it's good for catching legendary Pokemon that may be hard to catch.
Leave and return to the cave and Cobalion will have returned. So then you may capture it. I suggest Timer Balls.
TO catch my Azelf, I just kept on throwing 5 ultra balls, then 5 timer balls, then 5 ultra, then 5 timer etc. etc. (this is what I do with all my legendary Pokemon actually)
In all games its easiest with the master ball next if you can keep the battle going long enough the timer ball.
Grand Master Greevill has shadow zapdos to catch it weaken it to red hp then paralyze it use many ultra balls when your running out try timer balls i caught it with a timer ball.
Master ball or timer ball. Ultra balls aren't very effective against legendary.
Well it depends what Pokemon you want to catch groudon?
I used a Master Ball to catch Palkia, but that was before I knew Uxie, Azelf, and Mesprit were legendary. It all depends on the types of the Pokemon or how long you expect the battle to last. For Water-type or Bug-type legendaries, use Net Balls. For long-lasting battles, use a Timer Ball. For cave or night battles, use a Dusk Ball. (I recommend this for the Generation IV legendary trio, Uxie, Azelf, and Mesprit.) For Water-types, use a Dive Ball. (I'm not sure where you get this.) An overall ball to use would be the Ultra Ball.
you cant get a timer ball in emerald
Timer balls can be found in the Pokemart in Rustboro City