Some fights require more resources than others, of course. The githyanki patrol, for instance. I've never actually lost a fight against them, and I rarely bother with prep work or getting the high ground. But I do find that I lean on resources like potions and such much more heavily in that fight than others.
With Ethel, some folks say she's difficult, but my biggest problem is not killing her too fast in her house. I usually want to wait to kill her in her den. In fairness, I almost always have a magic missile spell at hand in that fight, making it much easier when her illusory doubles appear.
The Bulette is a rush to do as much damage as possible. I've never failed to kill the Bulette in the first three turns, but I admit it can be difficult, and I sometimes have to push myself. This is another fight where I rely heavier than usual on resources, specifically poison. It's also good to get some light or faerie fire on the creature, although I wouldn't call that foreknowledge. It's obviously difficult to hit; you notice that in combat right away, so it's immediately apparent that you need extra light or faerie fire to land enough hits.
I've heard people say the Minotaurs are tough, but I've never had a problem with them. I usually just concentrate all my damage on them one at a time until they've both fallen.
I always try to stay aware of my positioning, and I make a point of knowing what resources I have at hand. I think a big problem some players have is that they forget about their scrolls and potions and such. Part of what I mean about positioning is being cognizant of what threats are around you. Put yourself in a position that isn't attractive to your enemies. You don't want to tempt them into shoving you, and it's important to keep in mind that it's always better if your enemy dashes toward you instead of you dashing toward your enemy.
Use your effects. If you're trying to close in on an enemy, try to disarm them from range along the way. Try to frighten them, if possible.
Some people trade out weapons to take advantage of vulnerabilities. For example, they'll use blunt weapons on skeletons. I've never bothered with any of that, although it's probably smart to factor those things into your choices. For me, the skeletons fall plenty fast to a sword... fast enough that I don't have to worry about pulling out a club.
Which other fights are difficult? I suppose there's the druid grove. In that fight, I like to keep my companions close, but slightly separated. It's probably the best, most difficult fight in the entire game. Maybe a close second to Nere and the Duergar, assuming you don't have any help from rebel Duergar and you're not using any explosives to soften the targets.
Those fights are resource heavy. Concentrate on your positioning and use your inventory. Do you have any void bulbs left? If so, great. You can use those to reposition your enemies, helping to control the field. It's good to concentrate a few of your attacks so you can eliminate enemies from your opposition as quickly as possible.
A lot of it is just controlling your enemies and the battlefield as much as possible, all while remaining conscious of your positioning and utilizing your scrolls, potions, bombs, and poisons when and where necessary.
