Heya,
Just wanted to add my thanks and say that many of the posts below are spot on. We did this tonight and got 2 signets from the Sewers. We had 4 folks, lvl 79 cler, 79 bard, 70 monk, and 75 ranger. Bard or ranger tanked the mobs, so they weren't hard to tank with only 12,500 hps and 2200-2400 AC. If you don't have the signet yet (meaning, not raiding stuff), it's VERY unlikely you could solo. Duo is possible, but with adds, it would be tough. If you could handle 2 spiders or overseer robots at the same time in dragonscale hills, then you can handle going through this dungeon.
MOST IMPORTANT NOTE
As Tegwen said, "... when requesting the sewers instance, you want to make sure you get the one with the "Dragorn Foreman" and "Ukun Sentinel" for anguish signets."
This isn't obvious. When you talk to the Historian and tell him "Sewers" he actually gives you one of 3 quests. The ONLY difference is who they tell you might be inside. It MUST say 'Dragorn Foreman' and 'Ukun Sentinel' or you are wasting a LOT of time. Remove everyone and reget it. You can track from the round room near entrance to ensure the first is up, as mentioned below.
You can invis part of the way but will probably have to kill a couple at the traps. The floaty jellyfish guys see invis, so we just fought our way in at that point. Rumor is that Undead invis works. /shrug.
At the spawn place (as listed below), you should sit sort of in the water corner. That will give you one add. The 'a Cunning Dragorn' or his placeholder will pop every 20 mins.
Thanks to everyone for all their posts and good hunts,
Wyndaria,
Nameless.

Discolored Signet