I'm on the mission with the war chiefs and killing them.. 2 are down, but the others have killed me several times now and are really strong (resistant to stealth/range etc).
Any advice? I always get swarmed and die
If you're in a stronghold area, use the Caragor meat thing. I assumed those just attracted the Caragors already there, but that's not what those do. They call 2-3 from outside of the area. Also try fighting around grog or campfires and explode them (if you have the upgrade). If the alarm gets sounded, just run away (either out of the area entirely or up high in a spot you can crouch behind where they won't climb and wait for things to calm down.
The most difficult thing about any of the elite fights is when the unlimited swarms start happening. Otherwise they aren't that hard. Just exploit their weaknesses. Even at level 20 they still have a weakness. Before you can brand, just make sure you kill off the bodyguards of the Warchief first (not in that battle, but take them out in a seperate battle so they don't show up when you want to fight the Warchief).
You can sometimes lure the Warchief out of the stronghold area or in an area you cleared pre-battle (you should do this), but if they call in reinforcements with a horn or whatever, one of those will usually try to run off and sound the alarm. That's when the monotony begins and you basically have to reset the momentum.
It also helps to have the double special ability move unlocked, but that's the bottom tier, so I don't know how much of the side missions you've done before that main mission. It turns most fights into Wraith Stun > flurry (critical chains x5) execute, execute unless they are immune to combat finishers, which is one of the rarer resistances.