I just tried to kill a captain that was already difficult enough, and suddenly another captain appears that I already had killed. He taunts me, telling me that I should have made sure to kill him and swears revenge.
The appearance of that guy made an already far too hard fight impossible, and I simply ran away.
I'd really like for the captains I defeat to stay dead, how can I make sure of that?
To make sure that they stay dead, you need to behead them.
At least that's what the IGN Wiki tells me:
Death in the Nemesis System
Death is not the end in the Nemesis System: Not for Talion, and not always for the Uruks either. Whenever you kill a Captain or Warchief in Sauron's Army, there is a chance that they will survive the apparent killing blow. This can only happen if the Uruk in question was not beheaded in the finishing blow. Any Uruks that are lying down dead in the army page may come back and hunt Talion for revenge. After encountering them in the wild, they will reappear in the army page, now on their feet and sporting new scars based on their previous encounter. Some Uruks may even gain Strengths and Weaknesses depending on what happened to them.
Aside from beheading, making a Uruk captain's head explode will also make them stay dead after killing them (required skill: Wraith Finisher). With this skill, enemies that are killed by draining or Flurry Kill (on stunned enemies) will have their heads explode.
Also, you can feed a Uruk captain to a Graug (required skill to mount Graugs: Graug Hunter), which has the same effect of making an enemy stay dead.
Note that some enemy captains can't be beheaded / head-exploded by execution (e.g. those with the 'Combat Master' trait). In that case, you would have to make do with a drain head explosion (if they can be grabbed, grab them and keep on draining them until dead), Lethal Shadow Strike skill (if they aren't invulnerable to ranged attacks) or feeding them to a Graug.
I recommend actually planning the attack. Learn his weaknesses and strengths. If it doesn't show, experiment. And once you find his Achilles heel, try going one on one to enjoy it. Wraith Stun a particular Uruk and continually Flurry him, but never reach the end. If you have that ability to use 2 Melee Executions at once, stop Flurrying the particular Uruk and silmuntaniously attack 2 other unsuspecting Uruks. Or if you have a big combo going on, use a Wraith Flash, I think thats what it's called where you unleash a blue wave along the ground. It takes care of a lot of Uruks. Then after that, enjoy your one on one with the Captain. If there are multiple Captains, never let your guard down and use your brain before getting the glory. As for beheading, for me, throwing them off cliffs work as well but beheading was always the best.
