[Cerek]

I ALWAYS "Save" (or at least QuickSave) after ANY battle or killed opponent (if the game will allow it).

Your tactics sound very similar to mine in dealing with the duergar. I have gotten it down to a very specific routine. I lure the fighters away just as you did. I try my first HiS in the red room with the large "mineral/crystal deposit" in the middle. As soon as I exit the corridor, I duck around the corner and try to hide. In my experience, that area doesn't work very well. I then run around the big mineral/crystal deposit in the middle of the room and head for the room immediately to the South (where you fought the Lightning Mephit at the very beginning). I go around the corner to the left and immediately try to HiS again (had success much more often here). While running from the fighters, I usually pick one and fire arrows at him if I'm far enough ahead. Once my HiS is successful, I naturally target the injured fighter first (if there is more than one). Try to Backstab - then RUN as soon as the attempt is made and I become visible. If you do it quickly enough, neither fighter will get a chance to hit you. From the "Lightning Machine" room, I head for the cells where I first awoke in the Chateau. Run straight for the far end (where Jaheira and Minsc were). You should have a big enough lead to turn and fire a couple of arrows at the injured dwarf before they get too close to you. Make them follow you around the cages so that you can build up your lead on them again, and run back towards the Lighting Machine room. As soon as you turn the corner towards that room (coming from the cells), try to HiS again. This area was almost as successful as the top corner in the Lightning Machine room. If successful, then naturally try to backstab the dwarf that's already injured. After 2-3 backstabs, the first fighter should be dead. Then it is MUCH easier to do the same tactics with the second fighter. I just run in a loop from the Mineral/Crystal Deposit room to the cages and back, trying to HiS whenever I turn a corner and get out of sight for a moment. While running, I fire arrows OR darts (in order to save my arrows) at the duergar chasing me.

I don't know your alignment, but I chose Chaotic Neutral for my solo assassin. This gave him two "Vampiric Touches" in his Bhaalspawn abilities. I use these to kill the mage(s). It ignores their Stoneskin and can potentially kill them with one hit. If you don't kill them with that first attack, you can either try to use sniper attacks on them (and hope you keep making your Saves against thier spells), or just Reload and try the Vampiric Touch again. Yeah, it's cheesy, but so is the spawn rate of the duergar. If you choose to fight past their Contingency:Stoneskin, they will cast the following spells...

1) Confusion
2) Hold Person
3) Web
4) Melf's Acid Arrow
5) Melf's Acid Arrow
6) Magic Missile
7) Magic Missile

The first three spells aren't that bad. I immediately run for the nearest corner as soon as they start casting. That way, even if I fail my Save, I'm out of sight of the caster so he can't continue pounding me with spells while I'm Confused or Held. If he DOES catch up to you while you're are Confused or Held, he will blast you with Melf's Minute Meteors until the duration wears off.

After the first 3 spells, though, you will start taking a LOT of damage (from the Acid Arrows, Magic Missiles and/or Minute Meteors). You will most likely lose at least half of your hit points before you manage to kill him. That simply isn't acceptable to me, so I just reload and try my Vampiric Touch until I kill him.

Of course, if you ever manage to catch the mage alone and WITHOUT a stoneskin, then try to Backstab him before doing anything else - but this doesn't happen very often.