How can I get Mewtwo's Hidden Ability, Unnerve? I can be really useful in a battle.
You need GameFreak to release it in an event in order to get it. Mewtwo has not been released as of yet. Only 8 have been released so far: Tornadus, Thundurus, Landorus, Ho-Oh, Lugia, Dialga, Palkia, Giratina in the Dream Radar.
I don't want to ruin your answer Mewderator but actually there has been one event where Mewtwo has it's Hidden Ability Unnerve. Details about it can be found here:
Note: This Mewtwo was only available in Japan, Taiwan, Indonesia, and the Philippines through an event so your chances of obtaining a genuine one are very slim indeed. The only possible way would be through trade.