So I can't take credit for the build. I modified it based on Natalya85's "Fire Demon Hunter Speed Farm Guide" for T6 lower rifts.
Essentially the build works by spamming multi-shot and using the sentries to do the extra damage as necessary. It's an excellent room clearer once you start taking out mobs in 1 or 2 shots.
I only needed four pieces of the Marauder's Armor set bonus, plus Bombadier's Rucksack (+2 additional sentries bonus)