Midnight

Use your Dalaran Hearthstone, then take the teleport in the middle of Dalaran and head down the hall to reach the Return to Karazhan entrance.

Enter Karazhan and go right to clear the Opera event before proceeding to Attumen; use the Violet Seal of the Grand Magus before the fight begins, as equipping it mid-fight requires a 30-second delay.

From the entrance, go left, then right, then right again through the Grand Ballroom door, jump down, descend the stairs, and turn right to find Attumen the Huntsman.

Enter Return to Karazhan on Mythic difficulty and defeat Attumen the Huntsman solo; at 340 item level classes such as elemental shaman and frost mage can clear this boss without killing surrounding trash by using AoE.