So, I got to the last boss on Arcade mode and its bloody impossible! How the hell do you beat Unknown?? I just about got past Jun Kazama but Unknown is seemingly impossible. Anyone actually beat her?

Use Asuka and do her running kick thing (so, >> + X) over and over again. The AI just gets up into it. Jun has the same sort of move but it's not as good.

I played arcade mode at Medium difficulty and Unknown is kind of dumb defending up-forward launcher kicks. Then do a bit of air juggle and maybe do a bound to do a bit more damage. Try to kick her as far as possible then do a running kick thing. If she blocks, do a up-forward launcher thing again.

We had a thread just like this already actually. Just find a move that seems to work, and spam it, because the AI isn't smart enough to learn from their mistakes. Also if you're doing it to unlock the ending movies, it's probably easier to just play ghost mode and fight the gold enemies whenever they come up, after you win you will unlock the enemies ending movie and a customize item or two.