Pyrrha should work really well against Nightmare since she's the punishing queen of the game, but Dippy's Pyrrha only has one speed, which hurt him a lot. I'm not comfortable enough yet with Omega to have played that matchup well, and it seems like you have to really know Nightmare to play that matchup with Viola, otherwise you spend all your time getting pushed out of range and die trying to get back in. I managed to beat him a few times in casuals, but it was entirely dependent on my being able to get in and ride his ass till he broke.