The group I DM'ed actually managed to do everything else in the adventure before they ever found the roper, so they were already 5th level. Even so, they had it rough. The sorcerer got STR-drained to 0 in the first round, and the others managed to cut him free and retreat - past the roper and into the dead-end room. There, they waited a day so the druid could cast Lesser Restoration and make the sorcerer useful again (she didn't have it prepared).
Then, the sorcerer told the others, "Wait here, I'll take care of it!" He stepped out the door, and (using boots of speed to cast twice in a round) cast two fireballs at it (failed caster level checks), then two magic missiles in the next round (again, failed caster level checks, and he figured out he was too low level to even have a chance to affect it with his spells). I decided it wouldn't attack him immediately, because it wanted to lure the others out. So he panicked, shouted "Follow me!" and ran the length of the cave to escape. The others (foolish mortals) followed! They had to cut someone else free, I forget who, but they actually managed to run past it AGAIN!
When they returned, they had loaded up on Alchemist's Fire and other useful tools, planned their attack, and they managed to defeat it. But they only did so because their half-Duergar allies had given them some additional info about the roper once they knew it was there and went back to ask how to kill it. Once they were on the ledge with it, regular melee combat worked fairly well. By the way, I was amazed enough that my group managed to run past the thing twice without someone actually dying, and there was no chance at all that the PCs would have left the adventure without finding some way to kill the roper.
(About the half-Duergar - I changed that part of the module so that Durgeddin's surviving folks fled into the underdark, met some Duergar, got some sympathetic Duergar womenfolk to join up with them, and created their own home. When the Duergar found out where they were hiding, they sent some would-be-assassins after the half-Duergar group. The half-Duergar retreated back up to the Forge of Fury caves and made their last stand there, but were doing pretty badly until the PCs came along, killed the Duergar assassin party (not the prestige class, just a mixed group of Duergars sent to kill the half-breeds), and arranged to get them in touch with the dwarves that had initially told the PCs about Durgeddin, and who had offered money just for things with Durgeddin's mark. I did all this because it let the PCs use some diplomacy skills instead of just killing stuff, it let them do some political work with the dwarves outside the complex in getting them to accept half-Duergar descendents of Durgeddin, and it let me add some real complication to the pursuit of the Duergar would-be-assassins...plus the PCs didn't miss out on the big fight with the Duergar, it just happened under very different circumstances and against a group that was actually a good bit tougher - one of the Duergar killers, in fact, was the source of the Boots of Speed the sorcerer later used so poorly against the roper. I know, this isn't what you asked about, but it worked really well for me so I thought I'd share.)
I have a feeling that this will be the same scenario when they meet the Roper. Time will tell.
