So it looks like what was happening to my guys in Nurnen was due to the Bruz quest line.
In order to assure there's room in the hierarchy to spawn enemy captains during the Best Defense mission, the game will just keep killing off 99% of your captains in Nurnen somewhere around the time The Best Defense becomes available. (I'd sort of noticed it a bit before but it wasn't extreme so I'd just chalk it up to Nemesis stuff.)
Still, what a crappy solution. The game is about cultivating a stable of orcs. Murdering them without telling you why is bad. When the Best Defense mission showed up, it showed up with all the other Fortress Defense missions too. Did I run out and immediately start defending my forts, starting with Nurnen? No, I started shoring up my forts and my guys in preparation for the assault. Meanwhile my captains are being erased at a rate of 4 or 5 per cycle of one action, and I swear even grunts I'm dominating are turning on me until I finally do this fucking quest.
TLDR: as soon as you see The Best Defense just do it, if you want a chance at keeping any of your non-warchiefs/overlord captains alive.
I wish they'd stop handing out Blood Brother every other spawn. Every new orc that comes in can become a blood brother to one of your orcs, and once you control most of the hierarchy it's usually going to land on someone you care about. There's something weird with Blood Brother where even Dominating their Blood Brother and bringing them in to your army drives them crazy. Maybe it's intended, like seeing their Blood Brother humbled breaks your domination over them sort of like shaming does. I know Blood Brothers specifically get pissed off about shaming and deranging. That they get pissed about dominating too seems just a lil bit unfair though even if it makes sense. I'm losing guys constantly to Blood Brother, because they Iron Will up and I eventually have to Shame them so many times they end up crippled and likely to die anyways while I'm trying to get Iron Will off of them.
And while I really like the Fighting Pits, it's starting to feel a little like Salty Bet. On paper a guy should win but then the AI decides to potato the fight, and a level 34 Legendary gets taken out by a Level 15 Epic just by hits and counters, no exploiting weaknesses or anything. G'damn Defenders, I hate them so much.
Speaking of why everything feels like it's Defender Shields all day, I think I figured out how the orcs get designed, so to speak.
Every orc starts with a basic class.
-Warrior. (guys with a single weapon melee weapon.)
-Archer. (Crossbows.)
-Hunter. (Spears.)
-Savage. (Dual axes or swords or w/e. I think they also get some enrages from it.)
-Defender. (Spear and Shield.)
-Sapper. (Explosives.)
-Olog. (One large weapon and Olog Strength.)
From their basic class they get their weapon, and their general AI style, and/or a class bonus. (Ie. an Archer sits back while a Defender moves in close with their shield, while a Savage charges in.) This kinda represents their grunt training.
Then there are advanced classes, which they get when they're promoted to captain. All basic classes can become some of the advanced classes, but some are restricted by class and common sense. (Warriors without a crossbow can't become Marksmen, obviously. Ologs can't become Archers because they can't use a main ranged weapon, or Defenders because honestly, fuck that. (At least I haven't seen an Olog with a Shield and Spear.) I suppose actually Ologs can't be many of the classes because they can only use one main weapon, which is the super massive 1 handed weapon. Sappers aren't guys you typically run in to but it represents training with explosives which informs the Destroyer advanced class.
I'm sure this isn't how they're arranged but this is how I think of them.
Warrior: Slayer, Commander
Archer: Marksmen, Assassin, Trickster
Defender: Tank
Hunter: Beastmaster, Tracker
Savage: Berzerker
Sapper: Destroyer
So that's how you end up with awesome combinations like a Sapper who becomes a Berzerker. So they're a Berzerker with bombs and mines, and if they get immunity to their own trap damage type, they can wreck whole crowds. Savage/Berzerker means they get Savage Counter and will probably have a minimum of 4 Enrage triggers. A Sapper/Tracker gets mines, bombs and traps so they have the full compliment of placeable or throwable explosives. On the other hand, Archer/Destroyer or Hunter/Destroyer both get explosive shots which can destroy whole crowds of enemies or strip you of a lot of life. Defender/Berzerkers also turn out to be pretty nasty because they combine aggression with a lot of frontal defense and counters. Defender/Tanks are obviously pretty tough. Hunter/Tricksters are often pretty good because spears don't take long to throw so all the teleporting and dodging can be followed up with attacks. I'm not totally convinced Anything/Assassin is actually good except as a way to kill the player. Agile is cute and all but I see guys without Agile dodge each other all day in the Fighting Pits. Put another way, some classes seemed to be designed around making a player's life tough, while other classes are all around just better performers in the Fighting Pits. I love Slayers but, AI aside, I feel like they lose out to most other classes. Berzerkers seem to be one of the better classes in the FP, because they have lots of Enrage triggers and constantly attacking tends to overwrite the inherent shittinness of the AI. Ologs tend to do pretty well too just by dint of their strength. Archers and Hunters can be touch and go. Sometimes they zone their opponent the whole fight. Other times they get pulled in to melee and eaten alive.
In to this mix you throw the other modifiers like Cursed, Fiery or Poisonous and it modifies the traits underneath it to some degree. So when you end up with a Poisonous Hunter/Sapper with Poison-Proof they fill the screen with poison explosions all day.
I wish they'd been a little more creative with the Epic Traits, I feel like they could have done some fun things here. But most just make what's already in the game and its frame work more nasty.
For leveling guys, while I think the Arena gets you the most bang for your buck, it's super risky compared to doing Nemesis missions. But Nemesis missions tend to result in fewer levels for your orcs because you're farming up lower level spawns and investing time running around and death threating, etc....You also open yourself up to more Nemesis events, like get Blood Brothered because you're stopped paying close enough attention to the next orc in line to die, or you get preemptively ambushed by an orc you haven't met yet. Your other captains can decide to act on their own, complicating things sometimes because of rival or just too many damn bodies on the screen.
So I wish the Fighting Pit was a bit more reliable based on level. As it is if I'm trying to preserve my guys I level them up mostly on Maggot-tier fights, and even then I'd say I lose 1/5 just to AI that's deciding to derp around. It's a super convenient, entertaining and even profitable way to spend time in the game. I just get a little annoyed that it feels like there's a hard coded chance anyone can lose a fight.
I guess the moral of the story is that one does not simply level through Mordor.