Working on my poison kill achievement. Definitely using the Alchemist, Stealth, Poison Cloud, Poison Weapons, and Knockout Bomb. You could use Fire Flask instead of Poison Weapons or Knockout Bomb, though I find the bomb to be extremely helpful when soloing the RTC, as he is susceptible to sleep. I have a hard time getting the bigger enemies (Venatori Axe wielders, Red Templar Giants) to die in a cloud, so I tend to bring them down to 1/2 with a few melee strikes with poison weapons and that does the trick a bit better to get the killing blow with poison.
Definitely solo'ing routine, running thru the entire level, pulling everything around a corner/pillar, then dropping PC, watching everything die in the cloud while stealthed. If your positioning is bad to get them all in 1 of the initial poison clouds, I suppose using Fire Flask to drop multiple clouds at the start is good, but if a few die, their dead bodies drop more clouds to widen the AOE. That's why I like poison weapons over the flask to ensure the big guys getting poisoned and damaged a bit before I stealth.
Quit if it's Demons, and restart. They are smart, and move out of the cloud and it takes almost twice as long to get your kills via poison.