The official Japanese Pokémon Global Link site has announced a new promotional event, where a Gothimiru with the hidden Ability Shadow Tag and the egg move Mirror Coat will be distributed. The event can be accessed by entering a serial number that will be included with the first printings of the "Pocket Monsters Black 2 White 2 Official National Zukan Completion Guide" (ポケットモンスターブラック2・ホワイト2 公式ぜんこく図鑑完成ガイド) to be released August 11th, 2012.
The event will run until July 31st, 2013.

Source: http://ja.pokemon-gl.com/