Grohk using maelstrom can farm easy damage off of your team using Luna. Your best bet is to hide Luna behind a shield if you're capping the point with Luna. With totemic ward, he counters Luna's rush and your ult.

Unless recently killed, you can usually count on Luna to be somewhere near the fight, and for Io to be standing near her. Use your Shock Pulse on Luna for a free bounce between them. This might not apply if Io is using the Sacrifice talent.