This is part of The Terrible Titans… Lurking Locked Away! quest.
Leave Freezington via the exit nearest to the base camp. Cut through Frostpoint Field and turn left into the Giant’s Bed. The first Regi ruin is just ahead, on your right. Go up the steps towards the door. Remember to grab TR70 Flash Cannon on the right. Inspect the door to learn a clue: “Let ring the piercing note that will wake the giant of steel.”
Any ideas? Click down the left circle stick to whistle. If you did it right, the door will swing wide open. Inside, you’ll emerge in the Iron Ruins. Walk across the circular panels on the floor, until all 7 are lit up. If you walk over a lit up panel, the light will turn off, so pay attention to where your feet are.
The statue at the back of the room should light its eyes. Inspect the statue and select “yes” to challenge the keeper of the ruins.
Need a Regi-take?
Similar to the final encounter with Calyrex, it’s fine if you knockout the Regi Pokémon, lose the battle or choose to flee. The circular panels will all turn off. All you have to do is walk over them again to reactivate the statue. Then you can re-challenge the Regi (as many times as necessary).
All three of the original Regi trio are defensive-based, with Registeel having balanced physical and special defense stats. As such, outlasting the Regi shouldn’t be your main concern. Still, it’s a good idea to have ample healing items on standby.
Good Pokémon to send out are Electric-types to resist its Steel and Electric moves, although the latter probably won’t scratch you. For attacks, it can use Heavy Slam and Flash Cannon for STAB damage, Charge Beam, which starts weak but gets progressively stronger, and Iron Defense to rapidly up its defense.
If possible, start by inflicting Registeel with paralysis. Electric-types should be great at this. You could induce Sleep if you’d prefer, but you may have to keep re-applying it when it wakes up. Next, weaken it with any moves that tickle your fancy. Unless you’re overpowered, Registeel can probably take a lot of punishment.
Since Registeel likes to use Iron Defense, you may want to use special attacks to begin with, to bypass the boosted defense. Then switch over to physical moves to gradually chip away. Depending on how many times Registeel has boosted its defense, you might not even need False Swipe to get it down to low HP.
From there, there’s nothing to it but mindless Poké Ball throwing. The Regi all have low catch rates, so you may have to be patient. Be sure to bring plenty of good Poké Balls. Ultra Balls are great by default or you can mix things up by using Dusk Balls during the night or Timer Balls if the fight drags on, et cetera.
Catch Registeel and Legendary Clue 2 will be updated. Peony will also call you, to see how you’re faring. This will happen for every Regi you catch, except the last one.