First off, Runerigus isn't that good competitively. It is Untiered, seeing some PU usage as well. It doesn't do well in OU since it is completely outclassed by Landorus-Therian and Garchomp as an offensive Ground type. To make matters worse, all those two can be great defensive Pokemon as well. Now Runerigus has 2 serious and generally much better competitors. Oh, and Hippowdon. That's three. If you really want to use it then I'll try to make it work.
If you look at OU's physical move users, you'll see most of them rely on their ability. Corviknight relies on Pressure to PP stall Stealth Rock and win long-term battles with Roost. It also uses Brave Bird and U-turn, and occasionally Body Press. Corviknight does not like Pressure being taken away, since one of the things that make it such a great Defogger and defensive Pokemon is now gone. You can abuse Dragapult having Clear Body or Infiltrator taken away by Intimidating it with Landorus-Therian constantly, or set up Substitutes or Dual Screens in its face. Dragonite heavily relies on Multiscale to be as good as it is, so you can take advantage of that. Predicting correctly and switching into Sand Rush Excadrill's Iron Head means the opponent's Sand team has lost one of its greatest assets. Ferrothorn relies on Iron Barbs to be a good defensive answer to many physical attackers. You can take away Hawlucha's Unburden, and Runerigus is bulky enough to always survive Acrobatics. Kartana cannot destroy teams as easily with its Beast Boost gone. If Landorus-Therian hits you with a U-turn, it loses a lot of its defensive utility. Melmetal doesn't appreciate some of its power being taken away by losing Iron Fist. With Grassy Surge gone, Rillaboom is much worse of a wallbreaker. Tornadus-Therian, who according to most OU players 'never dies' will probably die soon if you steal its Regenerator. You can try to switch into Toxapex trying to use Knock Off but I would advise against it because Runerigus and almost all other Pokemon hates having its item removed.
Oh and in National Dex, you can use it with Slaking or Regigigas to get rid of their awful ability but it probably will not work. Mostly just a low ladder gimmick that fails at higher levels of play.
Runerigus @ Rocky Helmet / Leftovers
Ability: Wandering Spirit
EVs: 252 HP / 252 Def / 4 SpD
- Stealth Rock / Rest / Body Press