This is a special boss, that can be spawned by those who finished Zorra's Epic Mission Chain. In addition u need to possess some special items with u, to make it happen.
Ofc I could tell u what u need, but some ppl insist that i need to have some secrets here on this www... well, ok 😉
ps. just check Zora Mission Chain 🙂
Dmg type: seems to be Cold and Penetration but also a bit of acid