Arena is fucking hard... Running Yomi Dragon x2, Zeus Stratios, Awoken Haku, and Okuni. Everything hypermaxed, almost everything max skilled (Okunistillrefuses to gain that last skill up, im like 0/30 at this point) and best Ive done out of four attempts is floor 14. My favorite was my second run when I got to floor 9 I think it is that has the Shieldra's. Got two of the assholes that absorb dark damage for 10 turns and have the habit of popping hearts into darks. Guess how that went.
I guess the biggest frustration i have with it is just how varied it can be. You can do five runs and almost every single floor will be different. Hard to practice when your opponent is rarely the same thing twice.