If you're trying to do some LDoNs as a level 85 (I'm doing it for the Dodge aug), here are some things you should know:
The average level in the group should be level 83 or lower. Merc's count when computing the average.
Hard doesn't seem to be available. I suspect it is because normal at our level is already the max LDoN was programmed for, with all green and grey mobs.
You will get 105 points per mission.
I prefer the Single Boss type. You have to kill 20 mobs before the boss will spawn. This is obviously faster than the Mob Count type missions, which require 50 kills. I assume it is also faster than collection.