Runerigus

Such an amazing design, such wasted potential. Runerigus was never going to make waves in the PvE meta; its attack stat is just too low. However, while it was once dead-weight in PvP, the addition of Shadow Claw to its moveset in Go Battle League Season 12 has made it a worthwhile Pokemon in PvP.

Best PvE Offensive Moveset

• Shadow Claw is the best Fast Move for Runegrigus. 
• Astonish is a downgrade. 
• Shadow Ball is the best offensive option hands down.
• Rock Tomb and Sand Tomb match this ghost thematically, but they're basically worthless.

Best PvE Defensive Moveset

• Shadow Claw beats Astonish.
• Brutal Swing is the best overall option for a Charged Move.
• Rock Tomb and Sand Tomb both come out early. They are fairly comparable overall, but Rock Tomb pulls ahead a bit in general.
• Shadow Ball is usable, but it is easier to dodge

Shadow Claw is absolutely required for optimal performance, with a strong combination of damage output and energy generation. Astonish is a significant downgrade. 

Shadow Ball takes advantage of STAB to threaten major damage that's largely unresisted. Brutal Swing has great stats for pvp, and can put in a lot of raw power, but at times it's not ideal due to its nearly identical coverage as Runerigus' Ghost Type moves. Sand Tomb is recommended for its lesser energy cost and DEF debuff on the opponent, making it useful for baiting shields. However its damage output is pathetic if unshielded. Rock Tomb is a pretty poor move, and is only viable in specific formats where Rock-type damage is highly valued.