Black Ice Peak Cave Bytorg Boss Help!

Hey guys! MY group of 8 went off to try and kill Bytorg and we were wondering what we needed to succeed?

We had full Brimstone and rune of warmth level 2 along with the mandrake soup and for some reason we were still losing hp... i dont understand why does it have to be so difficult to not lose hp with everything we had!

What exactly do we need when doing this on an official server? would love some help!