Earthquake is a naturally strong attack, especially with STAB, and Runerigus has a naturally decent Attack stat. A purely defensive Impish Runerigus with Earthquake could potentially function. It does have some drawbacks, most notably being unable to damage Flying and Levitating Pokemon, but it's generally stronger than Night Shade, especially against Pokemon that are frailer and/or weak to Ground. Even if an Earthquake target forces Runerigus out, it still gives you an option to punish them for switching in that's more reliable than Will-O-Wisp.