Brick Break would probably not be that viable since Mr. Mime's Attack stat is horrible. If you wanted to go with a Fighting type attack, Focus Blast would be much better. Sure, not as accurate, but it will actually do some damage (and Focus Blast doesn't take two turns to perform). Even if you used Focus Blast three times and only hit once, you'd probably do the same amount of damage as three Brick Breaks. BUT, now that he's part Fairy... Darks aren't as dangerous, and if they were... Dazzling Gleam would take care of them.