I still think that the best classes to pair a nightblade with are soldier, inquisitor and arcanist. All are very good in adding dmg, OA - DA and in terms of defense.
Survival (especially in xpac content) is a mixture of a lot of factors. Trust me I’ve had multiple rips I understand how it feels, also had a couple of lvl 100 guys die so yea. First of all you need to have a very well balanced toon, meaning that he needs to have a great balance between offense and defense, you definitely don’t wanna focus just on one aspect, it is a recipe for a premature rip. Sometimes offense is the best defense and you definitely want to have the ability to kill as fast as possible, a dw nighblade is a perfect option for that. So aim for the highest OA with the highest dmg output and RR possible. For defense, you also got multiple options. The best are high DA, phys res and of course overcapped resists if possible.
You also want to throw some lifesteal in the mix, it will work great on a high dmg output char and you don’t need to have too much. With a very damaging toon even 5% will work great. Next pillar of defense is dmg absorption, you have quite some options in devotion, itemization and skills. Inqui has the seal, demo has blast shield, both are great options. Remember those 3 pillars that work wonders in combination with each other:
- lifesteal
- absorb
- high dps.
And when you encounter extra hard bosses you also need to have a DA number around 3k or even more if possible plus an overcap on most relevant resists.
When facing especially tricky nemesis guys and super bosses like mad queen etc. it is still possible to die if you don’t know their mechanic/being too reckless etc.
I’ll just give you some examples of bosses where death is quite a possibility even if you have a very solid toon but don’t know how to fight them:
- mad queen
- zantarin
- grava’thul.
Please don’t even attack those guys on ult if you don’t know what dmg they’re doing, what is most dangerous about them, what to avoid doing etc.
Yes it is possible to totally facetank all of those guys and people are doing it all over again with some very well optimized chars. But those are mostly sc players though, a lot of times you’ll see their hp pool dance like crazy, a little bit of bad luck and bam R.I.P. if you’re playing HC you should always open up your monster database before an encounter. Remember monster database is a HC player best friend because you’ll know what to expect and how to prepare.
And saving the best for last, always have a couple of aether clusters on a hot key, they definitely won’t suddenly turn a subpar toon into a S+ meta build with impossible greens, but can be a lifesaver because even the best character can have some bad luck sometimes. If you have enough of them you can sometimes just pop one casually when you know the boss is close to diyng and you just need that extra little push of 8 seconds to finish him. Also use oinments when needed, a lot of people are against them and I used to be one of them, but they’re in the game they’re totally legit and overcapping certain resists can be a pain sometimes, so just drink up when needed.
And in the end you can still die from not paying attention to f’in ground effects, happened to me once and almost again a couple days before. I just fell asleep behind the wheel for a brief second and walked my char into a nice aether pool, was just able to save her after menhir’s will kicked in and probably in a split second before death.
Yea bottom line, HC is not a very friendly place most of the time, also gearing can be a big problem in HC. The problem is, once you start playing hc you can’t really go back, at least I’m not able to. 
Forgot about one thing, which can be pretty important: I never quite understood why but for some reason Crate has a huge love affair with “shotguns”, you’ll see some kind of a shotgun (big concetrated dmg bursts) on nearly all most dangerous bosses ability lists. The usual mechanic is RR+a shotgun, so always pay attention when you are getting debuffed and try to just kite when that happens (for debuff duration). Certain bosses also have some kind of a retaliation bs (mad queen) and some will strip you of all your buffs in seconds if not carefull (grava’thul, “arcane” hero mobs). That’s why it’s super important to always know what the boss will be doing and prepare for accordingly.
didn’t mean to go off on a story but it felt pretty relevant to your experience and we seem to be of a similar mindset and approach to the game on hardcore.