NOTE: The only way in and out of Sulfras' Lair is through either Khoth's or Athron's Lairs.
Writing from the perspective of having entered from Khoth's Lair, look along the western wall as soon as you enter for a secret passage. There's three of these through the level, and they all take you to gates you can't get through. The large middle area is apparently Sulfras' private domain, and so far as I know there is no way to actually enter it.
Keep following the main passage around until it's turned to the west and you can see some portcullis' to the south. As you approach these will open, allowing you to enter and talk to Sulfras, who is sitting on an island in the middle of a pool of lava. There's a bridge over the lava to the south of Sulfras if you want to get closer to her, but you can talk to her just standing on the edge of the lava. She'll ask you to kill some Alien Beasts in a chamber to the northeast of her lair before she does anything for you.
Go to the northeast corner of the lair, where you probably noticed some barriers blocking a chamber. Take down the barriers and kill the Alien Beasts behind it. Check the room after you're done for a pair of Nimble Gloves and a Battle Frenzy Mind Crystal. Then go back and talk to Sulfras.
Once you tell Sulfras you've killed the Alien Beasts she'll tell you that she'll make a special weapon for you, but you have to go talk to Khoth to learn a ritual, and to Athron to get a lump of a special metal she needs to forge it.
Before heading back up to talk to the siblings, go over to the northwest corner of the lair. There's another room blocked by a Force Barrier here, with some Gorgons guarding some chests. Kill the gorgons and then open the chests. The first one you open will cause the appearance of a couple Gazers and Ur-Basilisks, as well as a Mung Demon. Once you've killed these bad guys check all the chests for a wide variety of items.
Now head up to the Lair of either Khoth or Athron, it doesn't really matter which order you visit them in.
Once you have the ritual and the metal return and talk to Sulfras again, and she'll make Beastslayer for you.