At early levels you will love blowing apart whole islands with Fireball. This fun soon stops though, as red mage towers become more prevalent and the attack power of leviathan unable to keep pace from around level 8-10.
Leviathan will likely be one of your first dragons that you breed, using Draco+Hext gives a 43% chance of success.
Maximum Attack: 4,545
A guide to using Leviathan