My group had their first session a couple of nights ago. There biggest problem was with the chocker in the theatre room in Jzadirune.
With the supernatural quickness the choker was able to grab a character and pull him down into the hatchway closing the trap door behind it. The players had a lot of trouble with the opposed strength check to open the hatch. It was pretty funny. Every so often the trap door would open and swallow another PC. I dropped 3 PC's to negs before they finally flushed it out.
That trap door just loved the taste of adventurers :)
I think my players would agree that the toughest fight thus far (we're about to finish up Test of the Smoking Eye) would be Aushanna- the party wasn't set up to deal with flying creatures, and she basically just flew about the statue and shot people for 5 or 6 rounds, dropping 3 or 4 PCs into negatives. They cast wind wall but it didn't do much good as she just used unholy blight on people behind it. And they never thought to run and take a defensible position. The only reason they lived is because it got around to 6 in the morning and they still hadn't killed her so I had her land and a few full attacks later she was toast.
Nabthatoron ran a close second, but only because they sped right through Vaprak's Voice without finding Alakast, didn't fight half the stuff so they weren't high enough level, and hopped directly into the Starry Mirror. Didn't kill or even drop anyone (expect poor Alek) with him though, he didn't think it was worth the trouble.