Well in my game there were only 3 trolls, and I'm not sure how you can beat that spider on impossible with that.
Plus, post a fraps cause I really got to see that
I mean, I tried the trolls on that fight and they work well, but they would still die before regenerating from attacks by the spider stacks. Perhaps if I had stoneskin and magic spring, you park them in a corner and just spell the spider to death... But I didn't have either of those spells.