Regirock got the short end of the stick: its specially based counterpart, Regice, is weak to a single rare special type, whereas Regirock is weak to Earthquake, Brick Break, and Meteor Mash. This does not hamper it as much as you might believe, however, as even an uninvested Regirock can survive Adamant Choice Band Medicham's Hi Jump Kick. Furthermore, it has crucial resistances to Normal and Flying, while Regice does not have any special resistances save Ice. With decent coverage between a STAB Rock attack and Superpower, Regirock can counter Dragon Dance Tyranitar, Salamence, and Gyarados—though watch out for Hydro Pump. Finally, if there is an enemy Pokemon your team can't handle, Regirock always has a trump card in Explosion to blast that opposing Pokemon out of the way.
Bring Regirock in against the likes of Tauros and Aerodactyl, on a move other than Earthquake. Focus Punch hits almost everything that resists Rock Slide at least neutrally; however, a notable exception is Claydol. Rock Slide should be used along with Thunder Wave, as this will give them a high chance to do nothing with paralysis and the flinch chance. Counter is a great move with Focus Punch, however, as it forces opponents into guessing games as to whether to attack, set up, or switch.
Thunder Wave should usually be the first move Regirock uses, because paralysis is always handy. Remember, Regirock's massive Defense allows it to take even STAB Earthquake and retaliate with Counter for the KO. If it lacks Counter, though, this set faces obvious problems with Claydol and Flygon, as all it will be able to do is use Explosion. Considering Regirock's nice Attack stat, it can blow up many threats.
The spread is simple and straightforward: maximize HP and Attack. An Adamant nature is recommended, but Impish is also viable thanks to its massive Defense. Note that, due to its titanic Defense and low HP, one should never give Regirock Defense EVs before investing in HP.
Simply switch this into a Pokemon Regirock isn't afraid of, such as Snorlax, and start setting up with Curse. Suicune or some similar Water-type will usually switch in to ruin your fun; this is when Regirock should Explode. A Curse-boosted Explosion will take out basically every Water-type, and Regirock can often survive a Surf from most of them. The other option is Rest, which helps Regirock stick around for longer. Rock Slide can help flinch a Skarmory using Whirlwind.
As usual, focus on HP and Attack for bulk and power. However, emphasis should be placed on Special Defense as Curse will boost Regirock's Defense to the point where, after a few Curse boosts, non-STAB Earthquakes will do a pittance to it. Do keep in mind that Regirock has decent Special Defense and can survive attacks such as unboosted Suicune's Surf and Zapdos's Thunderbolt.
While Regirock's stats might seem more defense-oriented, it does have some great attacking moves. Superpower works much like Focus Punch, but requires less prediction and can be used several times before Regirock is forced to switch out.
Hidden Power Rock is superior to Rock Slide on this set as it does more damage; Regirock is too slow to take advantage of the latter's flinch chance. In fact, the only notable Pokemon it outspeeds is Snorlax, who is hit harder by Superpower. The only reason to use Rock Slide would be to run Hidden Power Bug, which OHKOes Celebi, surprises other Grass-types, and does decent damage to Claydol. However, it is mostly inferior to the other moves.
Again, maximize HP and Attack, and go with Adamant for a further Attack boost.
Regirock's titanic Defense allows it to take Swagger-boosted hits from most Pokemon, and also screws with the enemy by potentially forcing them to hit themselves in confusion. Regirock can then use Psych Up to copy Swagger's Attack boost, and proceed to attack appropriately. Psych Up also works perfectly alongside Superpower by getting rid of Superpower's stat drops.
This set must be used with Spikes support; otherwise, opponents will simply switch out of Swagger at no cost and leave Regirock with nothing to Psych Up. With Spikes on the field, opponents will think twice about switching, especially as Flying-types such as Gyarados, who are immune to Spikes, face the threat of a super effective STAB Rock Slide.
Substitute helps reduce the amount of prediction needed to use Focus Punch; being naturally bulky, Regirock also makes a good user of Toxic.
Checks and Counters
Regirock's mono-Rock typing means it matches up poorly against many threats; it is weak to Surf, Earthquake, and a variety of Fighting moves. In particular, Claydol and Flygon screw it over completely; Regirock's only options are Explosion and Counter.