We've Been Dancin'

Travel to the top of Mogu'shan Palace in the Vale of Eternal Blossoms. If you lack flying, speak to Mishi, the small flying serpent at the base of Shrine of Two Moons or Shrine of Seven Stars on the bottom floor, and he will fly you up. Once at the top, speak to Lorewalker Fu and select 'Tell me of how Varian and Tyrande learned a Little Patience' to queue for the scenario.

At the start of the scenario, open your map and look for speech bubble icons — these indicate the two defense encampments assigned to you this run. You need the Pandaren encampment to be one of the two assigned encampments. If it is not, you will need to run the scenario again, as the two encampments are chosen randomly each time.

Go north of Varian and defeat the Pandaren on the lake, then have her accompany you to the Pandaren encampment. Her presence keeps the Pandaren awake after the defenses are constructed, which is required for the achievement to complete. Using only the water from the water sprite will cause the Pandaren to fall asleep once defenses are built, and the achievement will not trigger.