Things You'll Need
Instructions
Locate Regice by flying to Dewford. Surf north from Dewford, staying to the left, until you reach a large rock surrounded by a ring of smaller rocks. Enter the cave located in the large rock. If you are playing "Ruby" or "Sapphire," wait for two minutes inside the cave; once a door appears, enter it. In "Emerald," run around the the lap to make the door appear, and then enter it. These doors lead to Regice.
Go through the dialogue and initiate combat with Regice. Keep track of how many rounds go by. Hit Regice with powerful attacks until his health goes below 10%. Continue hitting him until he has low HP -- the closer to 1, the better. Do not kill him. Put Regice to sleep using the Sleep Powder skill on him. Fifteen Pokémon have this skill, including Bulbasaur and Ivysaur, who get this skill at level 13.
Determine which round you are in once Regice is at the lowest HP possible and is asleep. If you are still within the first 5 rounds, use your Quick Ball.
Continue putting Regice to sleep when he wakes. Wait 25 turns without doing anything if your Quick Balls do not capture Regice within the first five rounds. On the 30th turn, begin using the Timer Balls.
If you fail to capture Regice, load your last saved game and repeat these steps until you capture him.