This isn't a question on where the best place is to farm for SS, but the rate in which it will. I've been farming the 4s lizards for a little while now. I dropped 3 stinging swarm tome's in an hour. Do the tomes have individual % on dropping?
Yes. The shadowstrike tome is 5x less likely than any other sewer tome. Your odds of getting one to drop is about 1/1500 from any 4* mob, with less chance each * you go down. So kill 1500 of them to stand a 70% chance or so at at least one tome. Alternatively the drop rate from the two sewer bosses, ancient wyrm and elder revenant are better i think. The odds go up to only about 1/100. It's still faster to kill mobs for them but if the boss is up it's not a bad shot.
Now you understand why they easily sell for 50k+ on my server.