Barmanu is the Neanderthal Highlord. He is located in a cave after the Western Silk Road Portal. He is a very challenging boss for unprepared characters. However, with preparation, he can be one of the easiest bosses.
Barmanu has a high damaging default attack, in part from dual-wielding. One powerful attack is his "scream" which reduces life and stuns you for a short duration. However, Barmanu is infamous for his most lethal attack, the avalanche. He will call down an avalanche that moves fast and will cause massive damage, or kill unprepared (read: unmoving) characters outright.
Stun resistance is so important in this fight that anything less than 80% will just not cut it. This is because if Barmanu hits you with his scream, you will be stunned for several seconds, and if his follow-up attack is the avalanche, you will be slain as every single comet strikes you. But if you max Stun Resistance, you can get away before the snow hits you. Vitality resistance is also a plus, if you can get it. But Stun Resistance takes priority. As long as you have 80% Stun Resistance, just hit Barmanu whenever possible and avoid as many attacks as you can. If you don't have that much Stun Resistance (and at least by the time you reach him on Normal, it's not likely you will have that much), you'll need to kite Barmanu until he uses his scream--admittedly not easy to see against the snowy backdrop--then rush in and hit him until he telegraphs the avalanche, then run away and repeat.
If you have a pet, this can be used to bait Barmanu into using his stun/meteor combo while you are out of range, while keeping at a distance and using ranged attacks.