100 Poison resist per level of the mob wiill keep you from getting splatted.
level 40 mob, 4000 poison resist will keep you from getting destroyed. If it's level 52, 5200 minimum, and more if it's a raid mob.
Since I picked up resist gear, potions and form groups properly, I hardly ever get one shotted any more, and those 20k Nil's Devastation's now only hit for 2-3k