I'm bopping along through the game as a Gladiator, and doing fairly well -- I've hit the desert cities at around level 16, and this *&?% undead dragon is just unbeatable for me. Is that... normal? I can get it down to about 25% hits, and then I run out of healing potions. Reading old forums, the advice seems to be "stock up on poison resistance." Is this the trick? Because running around looking to replace all my stuff with poison-resisting stuff seems kind of... annoying.

I'm guessing by your post you have read Dragons post. While I usually survived these battles with range and magic, user: fangbite: found you could get up close and unleash special attacks with the Gladiator.
Rush up, strike and run away before the dragons main attack is unleashed if you want to go melee. It will take a while though :)
*edit: I also believe I know the one you are talking about, this one is a fairly hard challenge for a low level character, you may want to return when you have a wider selection of skills. The dragon will still level with you, but overall it will be easier.

There is a sure fire way to kill a dragon in melee, that I seem to be the only person in Ancaria to figure out.
Turns out dragons have really bad AI.
Get in to combat. Watch for its "tell", or, when it is about to use the breath weapon, as soon as it does so, break off from the attack and run in a circle around the dragon. The breath weapon will chase, but not catch you.
As soon as the breath weapon stops, go back in to fight, until the tell shows again.
Rinse and repeat. It will never realize that it could be guaranteed to kill you if it just stopped wasting so much time on its breath weapon.
IIRC I killed that dragon at around level 20 with my dual blade deamon.

Levelling up did the job in the end -- and a few health potions.
Now I have discovered that there's a bug in the game, and since I wandered into the desert dungeon before getting the official quest, I'll never be able to complete Mhic'Dar Mon quest -- kind of annoying. Apparently it's a game bug that never got resolved (and I went into the dungeon when I was roaming around a looong time ago, so I don't want to reload and start from there). No great reward there, but I wanted to bat .1000 on these things.

Which quest is that?
The game respawns.

Don't you have to pick up a scroll in that quest? If so, it only appears once you initiate that one, so there should be no problem with picking it up too early. Also, in most cases, picking up a quest item initiates the quest as if you talked to the person who would give it normally.
On the other hand, unfortunately, the game HAS some quest bugs... For example, main-quest related escorts can't die, they just pass out. However, in a recent game of mine, at a certain stage, one of them somehow managed to get themselves KILLED, which meant I couldn't progress in the MQ without reloading an earlier save.

No, you get a key from an orc shaman, run across to a dungeon, and pick up something for him. Unfortunately, if you've visted the dungeon first, the door that you need to open with the key will never open, even after reloads. I'm far beyond the region now, so it's not that big a deal.

Yeah, that's the quest I meant; it seems it's one of the more serious quest bugs left in the game.