If your having trouble killing the Beastmaster Warchiefs look to this guide for assistance.
Guide to Defeat Beastmaster Warchiefs
Tumhorn Beast Tamer
Tumhorn is the easiest of the warchiefs to kill, only requiring a single stealth attack in order to kill him (I like to stealth drain him because it makes his head go pop!).
Also a relatively easy warchief. Shoot him off of his caragor (they usually take 2 shots to the head to kill) and then vault over him, flurry strike him, then when hes on the ground do a ground execution. Do a combat execution on him 2 times, repeat until death.
Note: Make sure to kill the caragor that you are riding on when he comes out to make sure he does not kill it himself and regain HP.
Zugor Beast Butcher
If you have the ability charged, pull up the bow and press 2. You will be able to shoot unlimited fire arrows, killing him quickly.
If you do not have this ability charged then attack grunts till your combo reaches 5x, then shoot 2 fire arrows at Zugor. Repeat until death.
Ugakuga Graug Rider
Note: Do not shoot the fire place where ugakuga spawns before his Graug dies, it will be important for later.
First, kill Ugakugas Graug, this can be done by shooting it 2 times in the head and then letting the ghuls swarm it. Repeat until the graug dies, then get Ugakuga near the fireplace and shoot the fire place with an arrow, causing immense damage. Vault over him and flurry attack him. Ground execute him then do 2 combat executions, repeat until death.
Note: This can be tough to do due to the fact he will be constantly enraged thanks to the ghuls.
Shagflak Ghul Keeper
Let your ghuls attack Shagflak and brand more while he is being attacked, he will slam his shield down and exit, go outside the cave and catch up to him. Brand his ghuls and let them swarm him until the wretched graug comes, ride the graug and vomit on shagflak until he is dead.
Note: Make sure to eat ghuls while you are on the graug to heal or he will die.