I agree that there are not enough mobs in Unhallowed Essence dungeon, because no matter what grouping tools I try or how I position myself, I can't seem to complete the hit "X" enemies with one Multishot. I believe that there are just not enough concentrated mobs in this, plus the fact that they all die one shot. So after one fire, I can't even re attempt to group them up.
You kind of just have to get lucky. You can lure them together a little, but they are stubborn and it is risky time-wise. Sometimes it just doesn't work out. You have to make a call quickly whether there are enough mobs in a specific area, if not, go ahead of them, shoot backwards and wipe them out, and move on looking for a large enough pack. Typically if there is big spiders only in an area, you won't be able to get a 20 out of them. Just kill them and look for either 2 packs of little spiders, or 1 pack little + 1 pack big. Once you have the 6 "20 mobs hit" requirement down you'll probably find that you've accidentally skipped some stuff, so don't do that.
Get rid of all companions (and follower), turn off any rocket skills (choose different runes), consider even getting rid of your hatred generator to avoid accidental kills. I switched my generator to the ground-applied marked for death (it is also moderately useful for luring enemies). You will not be firing often enough to need it.