Objectives: Find Zaalbar
The lower sewers are permeated with mines, Gamorreans, and Rakghouls. The Gamorreans look ferocious, but aren't particularly strong. They can dish out a fair amount of damage with their vibroblades, though, so if you don't have much vitality, you'll want to track down Zaalbar quickly and let him be your furry tank.
From the entrance, head to the southwest. The first side-room you run across will contain a broken-down droid, as well as a corpse with the first Promised Land journal. If you Repair the droid, he will lead you straight to where Zaalbar is; if you don't have the required parts, just look in the nearby nest of rooms and you'll find him easily enough. You'll be facing a fairly tough fight with three Gamorrean elites and a Gamorrean chief, though, so it'd be worth your while to at least activate the droid, repair its weaponry, and send it on its patrol route; it should distract the Gamorreans long enough to let your party take them all down without getting harmed. Zaalbar is in the room off to the side of the Gamorrean hide-out.
Keep in mind that one of your three companions will have to be sent back to your hideout; since you have a soldier, a scoundrel, and a scout, you may want to choose whichever one overlaps with your class to be given a time-out. You can't remove Mission for story reasons, however, so you might end up doubling up on scoundrels if that's your class as well. Be sure to un-equip Carth if you remove him, since you won't have access to his equipment while he's gone.
Once you have your party set up, pick the lock on the chest in the room nearby to find Zaalbar's Bowcaster and other items. Equip him if he's in your party and begin exploring the Sewers. The only other items of quest-propelling interest are the second Promised Land journal, in a room in the northeast corner of the sewers, and the hidden entrance to the Black Vulkar base, which lies in the area to the east of our sewer entry point. If you wish to wrap up your Promised Land quest, grab the other journal and head back to the settlement; otherwise, go to the force field-protected base entrance and Mission will gain your party access.
Objectives: Get past the Rancor into the Vulkar base
The upper sewers contain the same basic threats as the lower sewers: Gamorreans and Rakghouls.
First off, head through the southeastern passage to find some items, then progress east across the level. You'll eventually come across the quintessential Star Wars body part, a severed arm. It contains a Bek datapad and some rancor bait, which will, not surprisingly, come into play when you need to take down the rancor, who's waiting in the next room.
The rancor, luckily, has his back turned, so don't worry about being stealthy here; just don't get too close. Walk over to the corpse pile, take the items, and then plop down a frag grenade and some of the synthesized odor back into the pile. The rancor will eat the grenade and get a sudden and severe case of indigestion. Step through the door, kill the energy-shielded Vulkars, and proceed to the Vulkar base.