I could use a hint on what I think is level 3 of the catacombs.
I've opened all the tombs, unlocking one of the two central tombs which is the passage to the next level down.
When I unlocked the final trigger, the "boss" skeleton who emerged was tougher to deal with than the previous ones. He moves very fast, and carries a bow that fires very frequently, with arrows tipped by a glowing ball. Strikes take alarmingly large chunks of health off. Worse, the guy chases you further and more accurately than any of the other boss skeletons. So, he's too fast-moving to have time to chip away at him at range with a crossbow, and frankly I'm afraid of close-quarters combat when just three strikes of the bow might kill me.
I ended up just going down to level 4, hoping to find a weapon that will kill him, or at least gain some easy experience and level up to the point where I outmatch him. Any tips on dealing with this guy?

It appears you need to toughen up a bit - see here: http://www.gamebanshee.com/divinedivinity/locations/catacombs2.php

Ah, I see. Like the "South Park" episode? To proceed, I have to go to the forest and spend tedious hours collecting tiny amounts of points in order to level up enough to get through the story. A pity. Thank you.
BTW, I like your icon. How sad that MTV isn't like that any more.

Something you might consider is using that Teleport stone you got as your vehicle for hit-and-run tactics. Admittedly, how effective this is depends heavily on what kind of character you play (mages or ranged fighters are best at this), but you could theoretically drop one stone on the ground at or near your target, and leave the other one near a bed that you're permitted to rest in. Fight until you don't think you can take anymore, then teleport away and rest. Repeat as needed.
What kind of character are you using? I took out the creature you mention as a mage without too much struggling... though it did take some patience...

I managed to deal with the Guardian with a combination of arrows and meteorstrike.
Running away periodically to recharge mana and health without being hit helped, but I didn't leave the dungeon just got out of his line of sight for a few seconds.
I hadn't left the village at that point and was using a survivor type character. As Prator says, just needs patience.

Thank you to everyone who made suggestions. I'm finding that the game pretty much requires the player to spend a fair amount of time in the salt mines.
By that I mean that once you get a Stats / Skills upgrade, new weapons, new armor, you CANNOT rush off to the next challenge. You have to spend the next two hours cleaning toilets. The first episode of this will be in the forests surrounding the Healer village where the adventure begins. You'll just have to wander around killing Orcs. Lots and lots of orcs. Over and over and over and over and over and over again.
You'll trudge back to the village to get healed, turn in crummy captured equipment for paltry amounts of gold, and slog back out to the forest. Again. And again. And again.
Why? Because this game is DEADLY. Unless you level up, and I mean level up a LOT, you can forget surviving. The enemies are too strong. There are too many of them. And they are too fast.
You will be overwhelmed again and again if you try to enter a new Quest or area unless you have put yourself through hours and hours and hours and hours of boring tedium killing what are at that time pushovers to you. Orcs. Trolls. Whatever.
It's like having to build a castle by picking up one grain of sand at a time with tweezers.
Wheeee! Fun.
NOT.

that's pretty much been the exact opposite of my experience. i've been waltzing through all the enemies easily, dodging them whenever possible because i don't have any reason to kill them. are you not doing the millions of cool optional quests?
also, what is your build like? consider getting a spell such as time cage to make boss type monsters significantly easier

Or he could get the freeze spell, I also find that poison is very helpful. Or try to keep a dagger that has stun, poison or freeze.

I'm playing a level 36 mage, myself, and no enemy can touch me if I don't want them to. My defenses include:
1. Freeze Spell. Invaluable for enemies who you want to hold still and who don't have high spiritual resistances.
2. Slow Spell (forgot actual name). Enemies who move like the wind or otherwise too fast for comfort can be dropped to a crawl with this, enabling you to easily run away, and it works on ALL enemies.
3. Skeletal Wall. Summon five skeletons to fight for you. They might not actually KILL anything, but damn me if they don't make a fantastic distraction while I move to a safe distance and start shucking lightning bolts.
4. Freeze Dagger. This seems to work even on enemies immune to the freeze spell, but I generally don't like to use it because it requires me to be within melee range. Oh well...
My strategy is generally just to obliterate the enemy with fire and/or lightning while keeping them at arms reach with the items mentioned above. I've taken down enemies twice my level this way.