My recommendation is to leave and re-enter the dungeon if you cannot get at least 2 or 3 20 hits by the time you clear the first big open area to the north east, since the entrance is fairly close by, which will save time since you don't need to Town Portal. The only difficult part of this dungeon getting monsters to cluster up so you can hit 20 of them with a single multishot.Similarly, I dismissed my follower and didn't use Companions as they also ended up killing the spiderlings. I would avoid using Vault - Trail of Cinders rune as it ends up killing the spiderlings that you want to bunch up.Generally just optimising for speed and resource cost reduction is the way to go. As long as you have the main Unhallowed Essence pieces and a Yang's Recurve, you can attempt this and it should be simple to clear (once again as long as you get mobs to spawn nicely).

The Demon Hunter's set dungeons are definitely the easiest I've attempted so far, and this one is probably the most difficult of the bunch, simply because it requires RNG to be on your side for monsters to spawn in a cluster so you can multishot and hit 20 mobs. You can find general Set Dungeon information and Set Dungeon guides for the other classes at my Set Dungeon Thread.

This effect rotates through the elements available to your class in the following order: Arcane, Cold, Fire, Holy, Lightning, Physical, Poison. Gain 150–200 % increased damage to a single element for 4 seconds. Critical Hit Chance Increased by 4.5–6.0 %

Easiest diablo 3 set dungeon