TM12 (Taunt) is on Victory Road. Smogon gives more detailed instructions than I can:
Go to the Victory Road entrance and follow the path until you arrive at the Veteran who slid down to the second floor level. Pass him, slide down to the next level, and head towards the easternmost entrance. Keep following the path until you exit at the rightmost gate of the third level. Head right, make it to where the opening in the fence is, and slide down to claim the TM.
