I just finished the Battle Arena in Circle of the Moon. I have to say that the easiest rooms were the Fox Hunter and White Armor rooms. All I do is slide at the hunters, and boom, they're dead. The White Armors...these are a joke. All they do is point the beam forwards, so I just keep my distance, duck, and start whipping. The Skeleton room was easy, too, but the Electric Skeletons and just kow many Skellies there were ruined my concentration. Also, the Adionbarg and Hyenas were pretty easy, but I still had the risk of getting hurt, unlike the White Armors and hunters.
The hardest room was definitely the Minotaur room. They're just so annoying, with their energy waves! I had to use the Mana Glitch with the Thunderbird summon just to beat them. Another hard room was the one with the Devil Armors and Evil Pillars. It just gets frustrating with so little space. The second hardest room was the Devil. He has a load of HP, high defense, a variety of attacks, and he did 120 damage when I was wearing the Dark Armor and I'm a level 55!
By the way, I got the Mars, Unicorn, and Black Dog cards in my first successful run, with only 500 luck! Yay!