Punkoinyc: With the exception of the Spy quest which I find somewhat challenging, the thief promotion quests are all about running past groups of enemies and getting out of dodge. Fighting everything can make them extremely hard, but invisibility or running for dear life seem to work wonders.
I think I've yet to manage the Spy quest without invisibility; there are just too many things that hit hard, soak damage, and punish you for hitting hard. Once again though, maybe I'm just not abusing the right spells.
I've waded through the corpses to finish the assassin quest recently, and it wasn't nearly as bad as I thought it would be. Usually I've used invisibility to dodge. Just running past everything doesn't seem to work; there are too many monsters that fill hallways too completely so you'd get stuck on something, then you are surrounded and pounded. Methodically murdering your way to the target lets you control the fighting.
I've usually manged to clear the monsters for the Rogue quest (slowly) using some AI issues. My problem is that someone is almost invariably at least KO'd (often someone dies) when I grab the quest item. Using the right spell would really make it into a run-in-and-out scenario though (assuming you don't mind leaving the highly trapped extra treasure behind, anyway). Invisibility seems a little dicey, with how little space the monsters have to roam around.