But he is supposed to be insanely powerfull. He is the son of the Deathwing, empowered by the red dragonflight, eaten the heart of the strongest mogu to ever exists and he has gotten empowered by the August Celestials. With all these things happening, it is not very hard to believe that he is insanely strong right now, and this is quite some time ago. Who knows what he has done since MoP to increase his own power.
Atm, he is proberly the strongest dragon alive. He has a strong feeling of ovnership over Azeroth, so he has proberly empowered himself since then so that he can protect the planet even better.