Pokémon Sapphire Cheats

I have found a method to catch the legendary pokemon, but first you need to enable the purchase of the timer ball at Rustboro City mart. You can do this by talking to the guy outside the tunnel entrance near rustboro (after you save him from Petalburg Woods). You can now buy Timer Balls and Repeat Balls from Rustboro mart.
Timer Ball: Increases chances of catching the pokemon if more turns are taken.
Repeat Ball: Increased chances if you have already caught that pokemon.

Back on topic. The repeat ball is unnecessary. Make sure you bring 5+ timer balls when you go after: Kyogre/Groudon, The Regi's, Rayquaza.

To keep taking turns, use the move False Swipe. Or, buy a load of pokeballs and use them. Make sure they are only pokeballs, they cost less and can count for a turn. the move False Swipe leaves the target with 1 hp without fail (however it is not instantaneous. Example, it may only bring him down to say...59. But keep using it until the bar is almost totally black.)You can keep using False Swipe because it will NEVER kill your target. You may want to bring extra potions and maybe a sleeper/paralyzer/freezer. You don't want the target to kill you instead. The combo I used was: Relicanth with Yawn (He was only lvl 33.) and Sceptile lvl 68 with False Swipe. Keep going with False Swipe and status problems. Then after a bunch of turns, try a timer ball.
A few more possible combos would be:
Swablu (Or a Sing-er) and A False Swiper.
Magneton/Manectric (Thunder Wave) and a False Swiper.

One final note. Do not burn, poison, or confuse the enemy. You do NOT want to kill him.