Start by killing all other enemies that can block you. Then attack him from the back. Once you hit him 4-5 times look for the signs ( Usualy his ROAR ) than press W+SPACE to dodge his attack and re-do this process untill he dies. I suggest only 1 player should be attacking him from close distance others can use arrows and magic to shoot him because you can end up blocking each other and that can make some serious problems. Anyhow i hope you enjoed this tutorial i hope i helped :)