Hey,
**spoiler space**
No specific 3.5 advice; just DMing advice:
If you anticipate some trouble getting your players into the Forge, have it be Winter time in your game world and set the Forge far away from civilization. This ought to be enough to get them "out of the cold" and into the Forge. (It worked for me, at any rate.)
When facing the Dragon, my players made liberal use of Walls of Force to create a "bunker" from which to delay and launch their own attacks. One of the Wizards in my group was even ballsy enough to Polymorph into a Shark and chase it down when it tried to swim away. One tactic I didn’t get to try was having the Dragon grapple a PC and take him under to drown him.
Lastly, don't forget about the Duergar ability to use Enlarge Person and Invisibility. When used in the smaller hallways and rooms of the Forge, these spell-like abilities allow the Duergar to both surround and surprise the PCs all at once.
Good luck!
J. Grenemyer
ps: I own Rappan Athuk, Tomb of Abysthor and Banewarrens. IMX, these three were pretty crappy compared to Forge of Fury. As always, YMMV.