In a double battle, any time I have my Braviary use Thrash, I dont have the option of selecting which opponent I want to use it on. when I checked any other of my Pokemon that had Thrash, same thing. Can I fix this?
Thrash, Outrage and Petal Dance are attacks that will attack any opponent randomly in double/triple battle. It's is not a glitch and it can't be fixed.
Source - Experience