Things You'll Need
Instructions
Assemble a team of Pokemon that includes a strong grass-type, a strong water-type and a strong psychic-type with sleep ability. You should also include some Pokemon with high HP (hit points) to soak up damage, as well as some weaker Pokemon that deal small amounts of damage.
Go to a Pokemart and stock up on great balls, ultra balls and restorative items.
Go to the Desert Ruins and initiate a battle with Regirock.
Use a strong grass- or water-type Pokemon to lead off the battle. Since Regirock is a rock type, he is weaker against grass- and water-type Pokemon.
Reduce Regirock's HP to roughly five or ten percent of his total health. The less HP he has, the easier it will be to catch him. An effective strategy is to switch to a weaker Pokemon that deals less damage after your grass- or water-type Pokemon have dealt the bulk of the damage. This way you can chip away at Regirock until he is down to only one HP.
Switch to a strong psychic-type Pokemon that has learned at least one ability with the "sleep" effect. It might take a few tries, but if your psychic Pokemon is strong enough, you should eventually be able to put Regirock to sleep. Pokemon that are asleep have a much higher chance of being captured.
Switch to a Pokemon with high defense and HP in case Regirock wakes up and goes on the attack.
Throw great balls or ultra balls at Regirock while he is asleep. It will likely take a lot of tries to capture him, so hopefully you brought a lot of balls. If Regirock wakes up and heals himself, or if he just heals himself, you will need to switch Pokemon accordingly so you can reduce his HP and put him back to sleep. Eventually one of the balls should stick and when it does, you will have captured Regirock.