A few people have run into glitches where certain opponents were invulnerable or just much tougher than they should be (more so in D2:ED than DKS). A couple people had this problem with boars, and several with one of the opponents on the mountain path at the start of the fjords. Sometimes just leaving an area and returning (to get the game to to reload the map) would fix the problem, or a couple people just went and did other things, and were able to take out the opponents normally when they came back later.
If the dragon stone is not disabled (it probably is), you could try returning to the battle tower and then leaving, and see if the opponents are still as strong. I don't think there is any way back into that part of the cave again after you take the teleporter out.
In DKS the first time I entered the Hall of Echoes, the flying opponents there were very tough (their attacks could not be avoided except at long range, and it took at least 10 Fire Spheres to kill them), and I tried that section repeatedly. I could not take out opponents faster than the nests produced them, so it was an impossible fight (even considering I had it on the highest difficulty). After I reloaded and did a few other things, the next time I entered the Hall of Echoes the opponents were still tough, but their attacks were not nearly as difficult to avoid, and 2 or 3 Fire Sphere were enough to defeat the common opponents (3-5 for the generals), meaning I could clear enough opponents to take out nests without dying, as long as I didn't make any mistakes. I also learned the shield skill, which helped a bit.
