Theory/Background - (The Why)
This build is made possible by the newly revised Hellcat Waistguard. The belt seems to give roughly a 50% chance to bounce up to 5 times, gaining damage on each bounce. I tested and can confirm that the damage bonus of 50% on each bounce and final bonus of 800% is indeed MULTIPLICATIVE with the original grenade damage of skill. The location of where the grenades bounce is random, but the resulting damage is huge.
This build uses Danetta's instead of Fortress Ballista with offhand. As a result you will have nearly permanent vault for extreme mobility. Also Danetta's leaves clones, stuns and knocks back mobs as you vault through them. This provides a great defensive benefit. Also since vaults will now cost hatred instead of discipline it will be possible to have max discipline at all times for more damage from the UE 6p bonus. As another benefit, vault will proc Focus/Restraint because it consumes hatred and the action shot rune deals damage. Just make sure to vault enough to keep up the F/R buff.
Gearing/Game Play - (The How)
Make sure to have at least 36.7% cdr for permanent Vengeance. Tactical Advantage passive is desirable to use for speed runs. If there is a reliable source of slowing then cull the weak can be a better choice than ambush in higher GR's. A hellfire amulet is preferred but a well rolled immunity amulet (especially arcane) is a fine choice too. For speed rifting Nemesis bracers are recommended if toughness allows. For pushing GR's use Wraps of Clarity. For bounties there is no better choice than the Warzechian Armguards. As with all UE builds, more discipline means more damage, therefore strive for maximum discipline on both weapons and cloak.
It goes w/o saying that it is imperative to keep up UE's 4p bonus thereby maintaining 2.5x toughness and 60% more damage (which is multiplicative with 6p bonus).
Make sure to spam Vengeance, bladed armor and wolf when off cooldown. The constant activation of these skills can be achieved by num locking them. Beyond that, all that is required is to vault for mobility and fire grenades to attack and regenerate hatred for more vaults. Its really that simple. You do need to be aware of your position relative to mobs and place yourself at least 10 yd away from mobs every 8 sec to keep up UE 4p. I personally use vault actively to stun elites and RG's to interrupt their special attacks. This can be be extremely helpful in surviving nasty fights. Note that while vaulting you enter into a smoke screen like state that renders you invulnerable to incoming damage for the brief time you're traveling from point to point.
Summary/Wrap Up - (The Results)
The build is very simple to play and a hell of a lot of fun. It gives insane mobility, good damage and high survivability. I have tested this build in season 9 on PTR in HC. With 800 para I cleared a GR 80 in 13:39 on the first attempt. This build will not be used to compete for pushing the highest GR's, but its more than capable of clearing well into the 80's with no problem. For speed farming it is superb. GR 70's are taking 4-5 min to clear. Normal rifts (T13 Rift Clear) are fast and fun. Split farming 4man T13 bounties is very doable once geared well. In short, outside of bleeding edge GR pushing (for that try Season 9 explosive-generator-94-grenade-build), this build can and will do everything you need and want within Diablo 3.
I have thoroughly enjoyed playing and testing out this build and I hope you will find as much enjoyment with it as I have. Please leave a comment below if you have any questions. Please Upvote if you find this guide useful. Thanks.