I've managed to beat Elpuri and Oree (and teleport away from the mage room) with natural limbs, which is pretty nice. I'm not quite confident enough in this character's stats to try fighting Petrus. There are methods of killing Petrus with mustard gas in 0.50.5; however, only one gas grenade spawned for this character, and it was on top of a mine. Ugh.
I spent all my wishes getting cloning charges and scrolls of change material. I couldn't even spare a wish for an amulet of elemental protection, let alone a grenade.
Before going for the shirt I tried to assassinate Petrus with just explosives - tamed all of the cathedral dwarves, stacked their backpacks on one of them, along with a few wands of lightning, fireballs, striking, and the only wand of acid rain in the game. When I aggravated Zak the kamikaze dwarf of Valpurus, he died partway through the explosion, and dropped all the unexploded items on the floor. They were whisked away to the dwarf room, partly intact - the wand of acid rain broke.
So, no mustard gas, and explosives are prone to failure. One option would be to give all the explosives to a friendly adamant golem and have a betrayed dwarf trigger them, but that can wait for another time. Looking at Petrus's equipment (and his entry in char.dat) reveals a total lack of acid resistance. Probably as a result of watching Orphan Black before going to sleep, I decided to try to give Petrus an acid bath.
Mellis spawns sulphuric acid rather often, and normally I dispose of it by dipping mithril weapons in it. This character managed to gather 53 empty bottles, and 34 more filled with disposable fluids. Based on my experiences making the sumo wrestler stand in vomit, my expectation was that the acid would eat through his boots of agility and destroy his legs, and the rest of his gear (which is all acid-proof) would protect him from further acid damage. I would then melee him with slightly more confidence.
A quick test in wizard mode revealed that this was very much not the case.
Ingredients:
* 45 bottles of sulphuric acid
* Tame armless rookie dark knight
* 3 Kamikaze dwarves of Valpurus
* 3 backpacks full of gunpowder
* Wand of slow
Initial experiments with an adamant golem showed that a broken bottle doesn't leave any fluid on adjacent tiles, so the acid would have to be moved under Petrus. While searching this forum for previous attempts to dissolve Petrus, I read another poster's experience getting an armless rookie dark knight to kick mines safely, and found that the same creature could kick a bottle of acid for 6 tiles without breaking it. I managed to get 37 bottles under Petrus's feet before he suddenly started kicking them a few tiles further. Facepalm.
Anyway, the stage was set. I brought 3 dwarves into the throne room, named one Zak and gave him the backpacks belonging to the other two. Manoeuvred them a bit, aggravated Zak and let him explode on the others.
The plan seemed to be sound; all 37 bottles broke, and all of Petrus's equipment was covered. A stethoscope showed that Petrus had also lost a little health. However, the stethoscope was in slot H when I tried to apply it. How terrible is that? I nipped outside the cathedral quickly so that I could drop the other tools and use a stethoscope from slot A. Much better.

When I came back Petrus was already dead. Even his dissolvable boots survived longer than him.
Repeating the experiment showed that it took less than 2 minutes for his head to be destroyed. This is clearly a winning plan.

To implement this in the real game, I'll need to spend a lot of time praying to Mellis, and probably stay out of the dungeon so that I don't get a nasty surprise from Sherarax or Ischaldirh. I've managed to disarm a veteran guard, whose kick seems to move a full bottle 6 tiles. I'd prefer a golem that can't train its leg strength, but I'll make do. I'll also have to clear all my other pets out of Attnam so that they don't interfere with the bottle-kicking phase.

I'll be back later with news of success or failure.

That sounds plausible. Might explain why it's my more powerful characters that have found ESPable golems, and my more troubled ones that have had ZQ-29 sneak up on them.
Strange that the game's attempt to compensate for the player's danger level actually makes the game easier in this case.

So for the first time I actually checked one of these ESPable golems with a stethoscope. Turns out the visibility condition hasn't changed, it somehow got bonus points of int, wis, per, cha. Which is still pretty mysterious. I zipped a copy of the savefile.
This doesn't seem to happen when summoning enemy golems, reading scrolls of golem creation, or polymorphing into a golem in wizmode. reading several scrolls of wishing will train your intelligence up to 5, but I don't think our jasper friend here did that.

I can't find the relevent line of code, but the cases of golems with int 4 and veterans with int 3 suggest that the minimum intelligence detectible by ESP is currently 4 (and anecdotal evidence suggests it's intended to be 5).
I'm not at all convinced that any golems exist with an intelligence other than 4 - outside wizard mode, at least.

They all have 4 intelligence according to the datafiles. The available ways of decreasing intelligence seem to be more or less incompatible with being polymorphed into a golem.
Creatures with 1 int like a veteran kamikaze dwarf or enner beast might work better - although their stats would be less impressive than, say, an invisible valpurium golem.
Oh, veterans have 3 int. They're still undetectable by ESP, though.

They don't behave as a pack, though.

Well currently golems are detectible with ESP.

Angband has (and variants inherit) a really evil social behaviour that hides in the corners of a room, and ambushes the player when they enter. That way they avoid being fought one at a time in chokepoints. When the player is low on health they become more aggressive.
It's not without problems - because of a combination of asymmetry in the line of sight code, telepathy, differences between LOS for vision and ranged attacks, and the fact that not all of the pack fits in the corners, the player can snipe a lot of the pack without retaliation before facing the "ambush".
With IVAN's more restrictive approach to ranged combat, that would be less of a problem.

The first time I saw the mage room, in wizard mode, with great gear, they killed me before I could make a dent in them. Since then I've favoured the overkill approach:
Amulet of elemental protection, as many other sources of electricity and fire resistance as possible. permanent invisibility, haste, telecontrol, phoenix feather and gef limbs, resurrected Sherarax, high endurance, dead or resurrected Ischaldirh, octagonal key, Turox/thunder hammer, lyre of charm, dulcis, scrolls of taming, explosives. Some of these are optional.
SPOILER ALERT. Click here to see text.
Not all characters get enough wishes/cloning charges for that collection to be really feasible, and the mage room is dangerous enough that so far I haven't even gone to GC10 in a normal game without being confident that I can handle them.
With natural limbs I don't know what the agi/dex requirements would be to take them on effectively. You need to be able to kill a mage and step on its corpse before a zombie is summoned.
I've never actually bothered to clear out the mage room - they teleport all over the place. I just kill the most dangerous ones to make sure the threat is neutralised.

In 0.50.5, the banana growers can still get stuck outside the shop window, which makes it easy to scitalk them.
Pets which are following the player won't try to stay next to the player, which makes scitalk more difficult the faster the pet is.
Pets which aren't following the player normally wander randomly, but past a certain number they form a clump which is difficult to break up (and fills the message buffer very quickly).
Monsters don't seem concerned by hazardous liquids on the floor. I haven't tested this, so it's possible that wise creatures avoid them, or that following the player ranks above pathing around environmental dangers.
Monsters use equipment, food, and drink from the floor, but not wands, scrolls, traps, throwables, chests, etc. That might be difficult to balance.
It's not clear what enemies can kick down locked doors, or what prompts them to do it.