Diablo 1 HD Hellfire Mod | Awake

Currently four of the my six classes reached Nightmare difficulty (Rogue and Assassin are still stuck in normal Terror's Domain for now). In this second difficulty, I ran into some difficulties I currently cannot figure out on my own and I was hoping for a hint into the right direction. They are independent from each other, but I did not want to open multiple threads within a short time.

1. My lvl 32 Warrior struggles with regular enemies in any dungeon lvl despite having max resistances and an Armor Class of 244 and 321 with 'Valiant Strike', but they are still manageable. However, sub bosses and quest bosses now have many modifiers and so not only their high 'To Hit%', which make my high Armor Class nearly pointless, hit me hard, but also the modifiers, each in its own way, take away my HP so fast that I can hardly attack. For instance, I was only able to defeat Zhar the Mad with a bow (duration: about 15 min), Lazarus with lvl 12 Lightning, and the Creature of Flame, where I was lucky and it was not moving, with Bone Spirit and Ice Blast (duration: about 30 min !!!)
Any idea for dealing with modifiers effectively without having access to the highly praised attributes like life steal, ignore target defence, and cannot be frozen for the Moment ? I haven't found any of those a single time in Nightmare difficulty, so apparently I cannot find and rely on those.

2. I tried to fight Nightmare Diablo a few times with the Necromancer. The problem is that he can take a lot of damage and whenever he has something between 75 and 50 % of his HP left after about 3 minutes, he suddenly disappears (in one case even in front of my eyes) and reappears at his spawn point with full health. Does anyone know why this happens and how to prevent it ? I already had this issue often in normal difficulty and was lucky that it did not happen in the instance where I finally defeated him. I am losing lots of time and potions because of that issue.

3. Is there a difference between Vitality and Hit Points other than that Vitality has synergy effects for the Necromancer or can be used to lvl the Insanity skill ? While Strength and Dexterity affect multiple aspects and Magic increases the 'To Hit' and provides synergy for some spells, Vitality seems to only increase Hit Points. From what I have seen, items that increase Hit Points directly have a higher bonus (+98 HP is the highest I found so far) than Vitality (I heard that +30 is the highest possible value, which would result in +75 HP for the Barbarian). My question is: Is there a reason to prefer Vitality over Hit Points ?

(4. Nothing important, but just out of curiosity: If you start the fight with Diablo in minute 59, will he receive the modifiers in the middle of the fight in minute 60, or can you still continue fighting Diablo without modifiers as long as you don't die so he will not reappear at his spawn point and have the modifiers for sure ?)

I am thankful for tips to any of the questions above.

Welcome to the forums!
1. It sounds like you might want to switch out some of your equipment for a package concentrated on improving your DEX and/or critical damage. Crits become super-important for a Warrior as the game progresses, especially in the later levels where enemy AC is so absurd the only way you're going to get any damage through (without ITD) is with them. I'm also really surprised you haven't managed to get at least some life leech going, usually it's not -that- rare.
Additionally if you haven't been spending much time on crafting now is the time to start. There aren't many good drops in Nightmare, especially for a Warrior, but the crafting recipes start getting pretty good. At least good enough to give you nice boost to your DPS, which it sounds like you need.

2. This is just a move Dibs has, although it is pretty easy to interrupt. He stops using it after he does his halfway-dead blow-up-the-pillars-and-minions move, so for both those reasons try to be as aggressive as possible in the front half of the battle... which is why it was happening to your Necro so much, they don't have the damage-dealing capabilities of a mage or physical-damage class at that point.

3. In the original Diablo, VIT also decreased the damage variables you'd take from spells, acid, and other form of non-physical enemy attacks (essentially anything not handled by AC).. but this mod made those forms of attack much more potent than they were in the original game so VIT's effect is pretty trivial here. In fact, I can't even say with certainty that that aspect of VIT carried over, it certainly seems like my characters take less damage with a higher VIT but that could my own confirmation bias.

4. That is a fantastic question. I will be trying that myself later and will let you know.

It appears the game has a bit of a problem handling the issue if you unleash Dibs and wait for the one hour mark to pass. My game crashed the first two times, but after I took the necessary precautions he did, in fact, immediately speed up at the stroke of the hour.

Nice. I didn't expect an answer to all of the four questions so soon. It took me some time to check what my private staches have and I did some theoretical tests by changing gear and checking stats:

1.1. I provide some stats from my current equipment in comparison with these when I go with 'Full-Dex' and use my best Dex Boosters from the trade stache:
STR: 138 - 138
Critical Damage: 128 - 128
DEX: 122 - 228
Critical Chance: 14 - 27
Armor Class: 244 - 89
To Hit: 207 - 327
Block Chance: 63 - 75
DPS: 259.88 - 225.65
Hit Point: 420 - 244

(Coincidentially, STR is the same with 'Full-Dex' equipment). I am concerned about my defence since there is a huge loss of AC and HP while Crit Chance does not seem to increase very much. I might find something in the private stache that can trade of a little Dex for a little more AC and HP, but finding high AC armor with a magical AC bonus is rare ; getting additional good attributes on it even more. The good attributes are mostly on low level equipment like skull caps or robes or occasionally on mails.
Two uniques (The Mangler, and The Executioner's Blade), have a huge impact on Crit Damage, but also decrease DPS a lot in comparison to Azurewrath.

1.2. Now that I have taken a closer look at my private stache, I found some life and mana steal from time to time. The problem is that almost all of them are on weapons, and no weapon I have have found so far gets close to Azurewrath in terms of DPS. The life steal items I have that are not weapons are a magical ring (2%), Bramble Ring (3%), Cathan's Seal Ring (6%), The Helm of Spirits (5%), and Leoric's Crown (7%).
I would trade attributes, resistances, and AC from my rings and helm for 16% Life steal, if I equip a Broad Sword with 6%, for 21% Life steal.
I tested it a bit. It can heal a bit with regular enemies. Fleshdom still Two-Hit K.O.'s me, though.

1.3. The only recipes beyond tier 1 I found so far are Wizard's Armor, Highlander's Amulet, and Upgrade Rare Weapon. Maybe Barbarian and Sorcerer (Rogue and Assassin if I manage to get them through normal Terror's Domain) will find more along the way.

I need more time to experiment. Hopefully I will find the optimal tradeoff/combination. Thanks for the answer.

Just as an addendum, that Wizard's Armor recipe you've got can be applied to any type of armor. I.. think they just forgot to make it apply to light armor only; either that or they figured it'd be pretty worthless if all you could use it on were rags. Whatever, it's probably the best of the armor recipes you're going to find for a while due to that fact.