Turtle

Breaking the unyielding is no easy feat, but haste is a cardinal sin. When faced with a formidable foe, it may be a prime opportunity to reassess your own path. Take your time to find the thread of your strategy.

And it has D-rank attack power, yet it's already around 33000 when it first appears.

Magic doesn't work and if your attack power is insufficient, it's tough, isn't it... Try to include a Medjed with the 'Wild' personality in your party, or use magic to push other beasts back and concentrate your attacks on Kurogane to make it easier to defeat (it does get slowed, by the way).