Team Plasma's Gone Too Far: To Gym

Slide across the ice north, then north again, and finally west to encounter a female Team Plasma Grunt. Battle her. Her Pokemon are Watchog (Level 44, Normal) and Muk (Level 44, Poison).

Slide west until you are in front of the Gym. Move through the gap between the ice crystal and the building on the east side of the path leading south, then slide south.

Move to the west side of the non-icy patch and slide north onto the non-icy patch with the final Grunt. Battle her. Her Pokemon are Golbat (Level 44, Flying/Poison) and Garbodor (Level 44, Poison).

After defeating the first two Grunts, head north back to the Opelucid City Gym.

Talk to Zinzolin, who is standing in front of the Gym, to begin a battle. His Pokemon are Cryogonal (Level 46, Ice), Cryogonal (Level 46, Ice), and Weavile (Level 48, Dark/Ice).

After defeating Zinzolin, Drayden will appear and give you a key item.

A member of the Shadow Triad will appear, followed by two more members. They will take the key item and disappear.

Follow Drayden, who will stop partway back to the Pokemon Center.

Continue back to the Pokemon Center, heal your Pokemon, and head east towards the gate back to Route 11.

Before the gate, a member of the Shadow Triad is waiting. Battle them. Their Pokemon are Pawniard (Level 46, Dark/Steel), Absol (Level 46, Dark), and Pawniard (Level 48, Dark/Steel).

Drayden will catch up after the battle, but the Shadow Triad member will teleport away.

You will receive a call from Hugh and talk with Cheren, who then appears in person.

After talking with Drayden, fly back to Undella Town.

Heal your Pokemon, then head west towards the Marine Tube entrance.

Just before the entrance, you will receive a call from Hugh. Speak with him, then enter the Marine Tube.