I have drained the sewers and have yet to beat the octothingy, but man I am having a blast with this game.
Does anyone know approximately how far I am into the game at this point?
Thanks

Aye... that frak'n thing was hard to kill. As soon as it got to melee range, it would eat me up in seconds. I ended up using the fire rune while backing up to keep it at a distance. When the power ran out, I would close and take a few wacks at it and then run back upstairs to the healing pool. Then hit the recharge circle and go back and repeat. I got the tentacle knocked off of it fairly quickly but the thing just refused to die. Finally after a few visits to the healing pool, I broke out the "special" dagger from the top floor and finished it off. I try to not use the dagger unless I'm to the point where I'm ready to quit. I'd rather use the normal weapons so as to get more hits in which levels your character abilities. I have a feeling that if you rely on the dagger too much, you'll end up with crappy weapon abilities when things start to really heat up.
As for your second question, you still have a loooonnnggg way to go :)
I just finished the 1st level and started the 2nd level of the mines which are below the sewers.

It is a hard creature to kill for sure. I found a fairly simple solution to killing him though, quite by accident while trying to get away.
WARNING: Spoiler
If you let him close right up on you and (here is the easy part) simply keep pressing the D or A keys you will constantly move in a circle and he will only hit you every so often. If you are fully healed it is likely you can take him out totally the first go around.
I tried it three times and I killed him 2 out of 3 times.

I think there are 15 levels and the sewers is level 4 or 5, so... ~30% done?

Thank you for the help, as it gives a better idea of where I am in the overall picture.

Man this game rocked and it has been so long since i played. I hated that creature. You will get to fight a few more too and some will be in close range with no way to run. Turning in circles is the best way to defeat it. Also just use the good dagger because when you get down to the dwarves there is a practice stone you can hit with stuff to level up the weapons. I remember equiping a weapon and putting a rubberband on the mouse button and walking away for a a couple of hours and coming back to have maxed out the level on that weapon.

considering the difficulty increase as you go further down and that the size of the levels will increase i would actually say you are more like 15% done

Wow!?! The levels get larger as I go; that is amazing and very nice to hear.
I am having a great time but man I need to find one of those Green Circles so I can heal. I am running on about 10% health and no help in sight.
Btw, I am currently on the level (I think) after the 2 headed person and the floor where there is a Brown Wooden lift.

There is a green circle on the 1st level of the sharga mines (the one with the snoring two headed ettin). It is behind a secret wall. The mine walls have a small circle in the pattern. There are a few places where the circle is actually a button you can press. If you already cleared the 1st floor, then use the mapping orb to find the hidden spots.

I have cleared the 1st floor but I cannot kill the 2 headed guy. As soon as he goes through his sequence I automatically die. I did steal all the stuff in the chest they were sleeping near though.
Is the secret door near them?

The secret door is about halfway between top/bottom of the map on the east or right side. It is very near where the hallway is blocked by rubble that you can't knock down.
If you looted the chest, you should have another orb. It has limited uses that slowly regenerate. Try clicking on your party with it and see what happens to your health :)

Oy! Those Throgs are a PITA. I finally came up with a way to handle them though (besides using the magic dagger). I hit them with the shrink rune which seems to make them fall a little easier. Later, on this level, I found a strength rune. The combination of shrink on them and strength on me seems to work pretty well.
There also seems to be an exploit that can be used in some cases (besides the spinning combat one). I have found that while falling back and ranging them, they will sometimes get to a point where they won't follow you. They basically stand there while you pelt away at them.
I have also found that the "octapus" critter can be shrunk too.

Thanks people, I was wondering what the green circle was for.
I killed the tentacled octopus thing by throwing arrows at it.
It seemed a lot easier task than when three ot those little white beggars come running up the corridor - RUN!!
I eventually came across the runestick on the second level down in the mines so killing the tentacled beastie in the sewer with magic must be quite a spectacular feat.
Regards.

I was informed in another post that the two-headed nasty is invincible - so break the barrels with missiles then walk to the chest.
Empty the chest then run away as fast as possible. - it worked for me!
Regards

When the going gets tough like that, I prefer throwing items, using an exploit. You can throw the item almost instantly after you close your inventory, then return to your inventory again almost instantly. Repeat the process and you'll wind up with what looks like a fire hose spewing arrows and axes, which is usually enough to kill any enemy, even if it takes just about every throwing weapon you could find up to that point. This can be a good way to train your Agility and Missile skill quickly. With those stats high, with the Short Bow in hand, you'll have no need for any exploits.