Moves
Stealth Rock is a terrific entry hazard, and due to Regirock's excellent bulk, it is one of the more consistent setters of it in the tier. Stone Edge is Regirock's most powerful STAB attack, allowing it to do the most damage possible to the Flying-, Fire-, and Normal-types it is meant to check. Alternatively, Rock Slide has higher accuracy and terrific synergy with Thunder Wave, as the two in conjunction create a paraflinch combination to wear opposing Pokemon down. Thunder Wave prevents offensive Pokemon such as Lilligant, Shiftry, and Klinklang from setting up on Regirock and has generally great utility in slowing down threats throughout a battle. Earthquake allows Regirock to hit opposing Poison-, Steel-, and Rock-types such as Garbodor, Klinklang, and Rhydon super effectively. Toxic can be used over Earthquake, as Pokemon that don't necessarily mind getting paralyzed, such as Mega Audino, Musharna, and Gourgeist, despise being inflicted with Toxic. Finally, Seismic Toss is a decent option which lets Regirock deal consistent damage to bulkier targets such as Steelix, Weezing, Ferroseed, and Piloswine.
Set Details
Maximum HP and 56 Special Defense EVs allow Regirock to avoid the 2HKO from Life Orb Pyroar's Hidden Power Grass after Stealth Rock. Similarly, 16 Defense EVs with an Impish nature let Regirock avoid a 2HKO from Life Orb Tauros's Iron Tail while also hitting a jump point. The rest of the available EVs are poured into Attack to allow Regirock to hit as hard as possible. Leftovers maximizes Regirock's survivability by granting it passive recovery. Chople Berry can also be used on teams that need extra insurance versus Fighting-types such as Primeape and Hitmonchan, as the item allows Regirock to survive even a Choice Band-boosted Close Combat from the two with ease and retaliate with a status move. Another option for Regirock is Custap Berry, which can net it some surprise KOs with Stone Edge or even let it get off a surprise Thunder Wave when under 25% HP. The item has excellent synergy with Sturdy and is fairly reliable, as Regirock gets worn down easily throughout a battle. Clear Body is the optimal ability for Regirock, as it allows it to block Memento from Skuntank and Mismagius, bypass Intimidates from Mawile, and avoid Special Defense drops from common attacks such as Shadow Ball and Psychic. Sturdy is also an option over Clear Body, as it allows Regirock to survive any attack from full HP. This can be especially useful when leading with Regirock, as it allows it to take attacks such as Samurott's Hydro Pump and Shiftry's Leaf Storm and proceed to either set up Stealth Rock or cripple the foe with status.
Usage Tips
Regirock should be used as a defensive tank that can switch into common Pokemon such as Kangaskhan, Tauros, Pyroar, Magmortar, Archeops, and Swellow and force them to switch out in fear of status or being KOed by STAB Stone Edge. Although Regirock does check all of these Pokemon individually, it will struggle to check several of them in the same battle due to its lack of reliable recovery, so only switch Regirock into these Pokemon when necessary. Beware of status coming from Pokemon Regirock can usually switch into, such as Will-O-Wisp from Pyroar and Charizard. If using Custap Berry, Regirock can trigger it by switching into entry hazards to get a surprise revenge kill. Once its HP is below 25%, saving Regirock for the late-game to get off a priority Thunder Wave is usually optimal. Do not blindly spam status moves; if the opponent has Pokemon such as Claydol or Steelix that is unaffected by Thunder Wave or Toxic, it is better to double switch into a teammate that can take advantage of these Pokemon and gain momentum.
Team Options
Grass-types such as Vileplume, Lilligant, and Ferroseed have excellent synergy with Regirock, as they resist the common Water- and Grass-type attacks often aimed at Regirock, while in turn, Regirock resists the Fire- and Flying-type attacks that threaten them. Regirock relies solely on Leftovers to stay healthy throughout a battle, so Wish passers such as Mega Audino make for great partners. Regirock can also easily switch in on Poison-types that threaten Mega Audino such as Garbodor and Skuntank. Psychic- and Ghost-types such as Mesprit, Xatu, and Rotom can help deal with the Fighting-types that threaten Regirock. Regirock is a very reliable status spreader, so slower Pokemon that appreciate Thunder Wave support such as Abomasnow, Kabutops, and Samurott work well with Regirock. These Pokemon in particular can all break through Steelix and Rhydon with ease, which otherwise give Regirock trouble.