This is part of The Terrible Titans… Lurking Locked Away! quest.
For our second Regi, we’ll continue exploring the Giant’s Bed. From the Iron Ruins, head straight forward, following the open space. When you reach the road sandwiched by broken brick buildings, turn right and follow the gentle slope downwards. After a while, cut through the tall grass towards the Old Cemetery.
Go around the Old Cemetery, preferably along its right side. Soon, you’ll emerge back at the Giant’s Bed. Ahead, the path will diverge left and right. Go along the path on the right, which leads slightly downwards. You’ll have to pass through some more tall grass. On the right, there’s a stream that leads to Ballimere Lake.
Keep going forward. There should be three large trees ahead. Between these trees is a path leading further in. Go this way and you’ll arrive at the second set of ruins. Clamber up the steps towards the entrance. Turn right and pick up the TR75 Stone Edge. Then check the door for your clue.
“Let the first Pokémon hold a never-changing stone.” This one isn’t too hard, right? Give the first Pokémon in your party the Everstone item to hold. There are many places you can find an Everstone–such as in Turffield. You can also craft an Everstone yourself by using the Cram-o-matic in the Isle of Armor (check the Example Recipes).
Do what the door says and it’ll open with gratitude once you inspect it again. Inside, you’ll arrive at the Rock Peak Ruins. You probably don’t need the instructions a second time, but just in case, walk over the circular panels on the ground until they’re all lit up. Then inspect the lit up statue at the back to fight the ruin’s guardian.
Honestly, most of what we wrote for Registeel is applicable here. The main difference is that Regirock is obviously a Rock-type. Also, it has obscenely high physical defense and decent special defense. If you want to deal damage, prioritise special attacks or super-effective attacks. Or you could be here all day.
For attacks, this rugged Regi can use Stone Edge, which can cause a fair amount of damage (especially if it criticals), but has low accuracy. It can also use Superpower, a strong Fighting-type move that lowers its Attack and Defence, or Hammer Arm, which lowers it Speed. Finally, it can use Curse to lower its Speed in exchange for Attack and Defense.
Throughout the fight, Regirock will most likely get slower and slower (through Curse and Hammer Arm), which doesn’t really impact it much. Since it’s super slow already. Its Attack will probably stay roughly the same, until it runs out of Superpower (5 PP) and its Defense will generally have a net increase.
Fighting and Ground-types are great for resisting Stone Edge; Ghost-types will laugh at its Fighting-type moves. A Ground and Ghost-type like Runerigus would be perfect here. But one or the other should be sufficient. A Pokémon with the Pressure Ability could be useful to make it quickly run out of Stone Edge and Superpower (but switch out later so it doesn’t Struggle to death).
Once you’re in a good position, where Regirock has low HP and is paralysed etc., it’s time to throw those Poké Balls like your life depends on it.