This build is aimed at 4man farming / pushing, playing as a semi-support throughout the rift and killing the RG very fast. It's a little better than Gen Monk on lower tiers and roughly the same on high tiers (115+) while bringing better buffs and more utility in general. The reworked Holy Point Shot (Impale now throws two additional knives) makes this build good in patch 2.5.0.
Full Shadow's set, Karlei's Point, Holy Point are required, ideally you want to supplement that with Witching Hour, Lacuni Prowlers, Stone of Jordan and the Endless Walk combo. Roll CDR on your shoulders, attack speed on gloves and perhaps Compass Rose, stack elemental damage + chc + chd where possible. Try to get Impale damage on boots, helm and quiver. If you can't get a good amulet / SoJ / HPS with lightning, going with another element is also a possibility, but a little suboptimal for the rift.
D3planner link with optimal rolls: http://www.d3planner.com/705178239
Depending on the tier you are running, you either want to play as a hybrid support during the rift, meaning you will buff with Marked for Death - Contagion (try to aim at low hp targets with it) and your Wolf Companion, and try to help the Barbarian out on the pulls. At very high tiers your damage will become pretty marginal, but up to 105 or so you will be able to finish off some low hp elites. On the RG, make sure you stay at least 10 yards away (for Steady Aim), use your Wolf Companion so that you get it on every Convention cycle, and try to position yourself so that you always hit with all three knives. If the RG spawns adds, you have to be very careful about where you are standing, in most cases you should probably just give up Steady Aim and stand right on top of it.
Instead of Stone of Jordan, you can go with Elusive Ring for a big toughness boost (for farming or hardcore), and instead of Leoric's Crown you can go with Aquila Cuirass (same reasoning). Bane of the Powerful can be used instead of Gogok on lower tiers where the RG dies in 1-2 min. Shadow's can also be used for solo pushing or farming lower tiers / torments, I will cover that in separate guides.
Feel free to watch the video for more in-depth explanations and the reasoning behind some of the item / skill choices etc.