[Menarker]

That or the aforementioned Naqel/Raptor using Endeavor (Naqel can also use False Swipe to safely reduce it to one hitpoint.). Besides, once we get Genom by itself, we can relatively safely have Rio kill the one summon and does every turn until we weaken it little by little or get it to surrender. Togekiss can be revived if it gets knocked out.

The Starmie idea (Paralyze the one that Rachel would have paralyzed) sounds better since Registeel is a legendary and it's power combined with a psychic attack will help either kill it (optimistic result) or sync-crush it (likely result).