Running is always an option, monster placements are fixed and you will often encounter guys who are just badass compared to the average encounter level.
My first time through as a warrior, I cruised through the first few levels of the introductory dungeon. Until the last level.
I HAD to leave the skeleton conjurer lords well enough alone. It grated at me and I certainly came back later to off the @#$%ers. That and the poisonous giant spider beneath one of the special floor tiles. Tough son of a bitch.
As a mage however, the first spell I picked up was hell spikes. Its instant effect, cheap to cast and you can spam it via repeatedly clicking, it also breaks their attack either by them trying to avoid the spikes, or better still by stunning them with damage (like when you're hit by an arrow).
Once my mage got outside though it was back to square one. The harder orcs just laugh off all of my spells because they have the hit points to soak up their meagre damage.
As your character grows their offence and defence goes up and they get game altering skills (like the extremely overpowered freeze spell). You can use these to balance the scales or just go with the flow and tackle a different area until you're stronger.

Another awareness is that while holding the control key down in battle . . . your F key link to your healing potions won't work.
You will find yourself standing there fighting away, tapping the F-1 key (or whatever you have linked to your health potion), CTRL key pressed to focus on the closest enemy . . . and dying without knowing why your health didn't go up.
So . . . keep an eye on your heath bar and let off the CTRL key to insure your health potion is utilized. I really like the CTRL key option as long as you realize how it works . . .=)
Edit: The CTRL works really well in the catacombs against the lesser enemies, holding down the CTRL and left mouse button, your character will go from one to next without stopping . . . just watch the health bar and let off to down a potion.
Oh yeah, if you have the CTRL key pressed . . . you can't run away . . .

I've found for the starter dungeon, the best bet is to make use of the bow early. You're too weak in my opinion to do really well at melee starting at level 1, so take the easier approach to take out all opposition. Once you get out of the starter dungeon and past all of the outside orcs, then I'd think about developing melee and magic capabilities.

Thanks for the advice. I manged to make it through as a Survivor with a little help from the pause button and as many potions as I could buy. Once I got about half way through the 3rd floor of the Catacombs, the rest was a breeze.
I'll be back for that damn Giant Spider later though. He was the only one that did manage to kill me.

The starter dungeon has a few enemies that are very tough for the level you'll be at that don't need to be killed, so there's no shame in coming back to deal with them after gaining a few levels (also, the game doesn't put many hard limits on where you can go, so you'll often have to use some discretion in deciding what to take on). Also, when fighting enemies in more open areas you can sometimes beat tougher enemies by landing a few hits, then kiting them while you heal up, rinse and repeat. However, the game does get significantly easier once you start gaining some levels and can actually start customizing your character to better suit your play style.