Note: I was hesitant to use Arceus' Z-Buff on round 3 (Dispells diving pokémon and makes Arceus move again), because I wasn't 100% sure if that would trigger Resonance Ho-Oh. On a rematch, we confirmed that Arceus can indeed just use his Z-Buff, move again and kill Palkia before Resonance Ho-Oh has a chance to move :)