Monster generation?

What factors determine exactly what monsters are being generated? My latest character was doing well after going through Alveradok's natural training regime. He still only had 10 endurance while he was going through the gloomy cave, so when he found an armor of great health +3, he immediately donned it and hardened it to ommel hair, since it was both better than what he had and gave him some much needed HP. As expected, the monsters he met from that point onwards were stronger, but for a while it was manageable. Eventually, with his endurance naturally increasing too, he got to ~98 HP altogether (still all natural), but after starting to meet adamant and octiron golems wandering around GC4-5, he swapped the great health armor for Atavus' arcanite plate mail, hoping this'll help the situation.

However, the monsters generated were no less ferocious even if the character's health plummeted to 54. It later crept back up to the 70-80s naturally, around when I decided to plow through GC6. Donning his cloak of invisibility, the little guy prepared to waltz through the level when the game suddenly decided to arbitrarily kill him. Golgor Dhan and Ur-Khan were generated simultaneously. Despite having 30 perception, the enemies seemed to be able to follow him even when they were outside his massive LOS, so he started teleporting away. Then something invisible attacked and severed his left arm immediately. His arm and its contents were flung about 7 squares away, so he decided to teleport onto it, grab the stuff and then GTFO, but the invisible thing was too fast and lopped off his other arm next, then killed him twice in quick succession.

The highscore indicates that he was killed by Experiment ZQ-29. This is the first time I've ever encountered the golem in a non-WM game, and frankly, I do not understand how with endurance amounting to ~80 HP I get one of the most dangerous bosses of the game, and then two others who are just as impossible to kill. On that subject, I'm not even sure how to deal with these bosses. I've never managed to beat any of the bosses that are stronger than Xinroch. I had quickly figured out that meleeing Ur-Khan and the like doesn't work, and wands don't kill fast enough either.

END is particularly bad for the danger level, yes.
Equipment you use can also lead to a much higher danger level, with certain items being worse for it than others (particularly artifact items) and increasing the danger level the more powerful they are.

IIRC actually, all stats contribute to a higher danger level, it's just that END is the number one offender in this case.
Kinda sucks the game spawned not one, but three named monsters on you at once at such an early level. I don't suppose you had some ridiculously buffed equipment?

the AoLS is also a huge danger level increaser in my experience.

Both of those guys have better Perception than you + Infravision, that's why they were able to follow you. Others might have ESP + high intelligence.

70-80 HP with good equipment seems like enough for them to be spawned. The game I was just playing, I think I had 87 HP and Sherry was spawned and killed me before I even realized she was there (I had phoenix feather & mithril equipment, 2 whips of thievery +11, and ommel hair limbs).

In my experience, bosses are usually spawned at a point BEFORE you are ready for them. Where you would die instantly if you tried to take them on. There would be no challenge if you were ready for a boss when s/he first appeared.

Best instinct is to run, hang around the previous levels for a while, then go back. Lots of tactics on the forum for dealing with them, I usually go with a row of mithril bear traps.

What counts as "ridiculous"? I found three wands of mirroring and one scroll of charging, which allowed me to boost myself up to a mixture of ommel hair and phoenix feather armor, but none of the boosts were higher than +5 and some were lower.

Point taken about the AoLS, although I think I'm still gonna wear that thing whenever I find it. The ESP amulet couldn't have helped me see the golem anyway.

on the handful of times I found an AoLS I only wore it when fighting serious battles to avoid overspawn. except for the time I took on richel decos but left the aols in a chest.

Invisibility is also a major offender. Not only does that allow you to hit & run, the game adjusts the difficulty so you have to hit & run.