Garrosh Hellscream is the LAST encounter in the Siege of Orgrimmar and the DOWNFALL (YEAAAAA BUDDY!) LFR.

For a detailed view of the abilities and loot table for LFR/Flex/10/25 please CLICK HERE

General Strategy/Overview

This is a multi-phase encounter, with each phase transition happening when Garrosh hits 10% health.  At which point, he will heal himself to full, gain new abilities, and no longer summon adds to assist him.  During the transition phases you must fight your way to Garrosh where he is trying to absorb energy from the Heart of Y'Shaarj.  For every 25 energy he absorbs, he will empower one of his abilities, causing them to become even more devestating.  When he hits 10% health a 2nd time, Garrosh will fully drain the Heart and gain 100 energy, fully empowering all of his abilities and healing ~20% extra health. At this point you just pop hero, nuke and hope you win.

Phase 1 - The True Horde

This phase is about add control (go figure) and surviving all of Garrosh's abilities that throws at you.

Garrosh

1. Desecrate - Garrosh throws his Desecrated weapon at a random player dealing ~160k shadow dmg to all enemies within 15 yards/1 sec, and leaves a Desecrated Weapon at the location
   1. Desecrated Weapon - This leaves a large void zone surrounding the weapon, destroy the weapon.  As the Weapon loses health, the smaller the void zone will become.
2. Hellscream's Warsong - Inspires his allies, increasing their phys dmg dealt and health by 200%.
3. Hellscream's Allies - these adds come in throughout Phase 1, and will progressively join the battle faster the longer the fight goes on.
   1. Kor'kron Warbringer - Melee mob
      1. Hamstring - deals 150% of normal dmg and slows player by 50% for 10 sec.
   2. Farseer Wolf Rider (HIGH PRIORITY) - Shamans that have a relatively high health pool, must be focused quick to prevent raid dmg and add healing.  All abilities need to be interrupted.
      1. Ancestral Fury - When interrupted, gains this stacking buff, increasing 25% cast speed per stack.
      2. Ancestral Chain Heal (Interruptable) - Heals add for #% of his max health, chains to nearby adds.
      3. Chain Lightning (Interruptable) - Deals ~200k nature dmg to target, chaining to nearby enemies.
   3. Siege Engineer (#1 PRIORITY ADD) - These adds appear on the balconeys on the sides of the room, and if not killed within 15 seconds will activate the Kor'kron Iron Star
      1. Kor'kron Iron Star (wipe mechanic) - Deals MASSIVE dmg to any player hit by it.  When it hits it will explode dealing ~3.4-3.5Mil fire dmg to nearby enemies.  Deals less damage to players the further away they are.  These can be used to kill any of Garrosh's adds that are up.

Interrmission: Realm of Y'Shaarj

When Garrosh reaches 10% health, he will begin to start siphoning power from the Heart of Y'Shaarj.  This will heal him to full health and grant him some new abilities.  At this point Phase 2 starts and periodicially throughout the phase, you will be transported into the "Realm of Y'Shaarj" where you will have to run a slight gauntlet to get to where Garrosh is continuing to siphon dark energy from the Heart.  You want to reach his location and AoE down any remaining adds ASAP to stop him from siphoning energy.  Once Garrosh reaches 25/50/75/100 energy, he will fully empower one of his abilities for the remainder of the fight. 

Realms

1. Terrace of Endless Spring
   1. Embodied Fear - Adds that sustain Y'Shaarj protection on Garrosh.  Once defeated, Garrosh will stop siphoning dark energy.
2. Temple of the Jade Serpent
   1. Emboded Doubt - Adds that sustain Y'Shaarj protection on Garrosh.  Once defeated, Garrosh will stop siphoning dark energy.
3. Temple of the Red Crane
   1. Embodied Despair - Adds that sustain Y'Shaarj protection on Garrosh.  Once defeated, Garrosh will stop siphoning dark energy.

Garrosh Abilities while in the Realm of Y'Shaarj

1. Y'Shaarj's Protection - while any of the Embodied adds are alive, Garrosh is protected by this and will siphon energy from the Heart.
2. Annihilate - Once the protection is broken, Garrosh will spam this ability.  This is a frontal conal abililty that will deal high dmg to anyone infront of Garrosh.  Will also deal moderate raid-wide AoE dmg.  This will continue until the end of the 1-minute timer that you are in the Realm.

Phase 2

Periodically throughout this phase, until Garrosh reaches 10% health again, you will be re-transported to the Realm of Y'Shaarj and have to run the gauntlet again to prevent Garrosh from gaining more energy to empower his abilities.

His abilities for Phase 2 are as follows

1. Whirling Corruption (DEADLY) - Garrosh begins whirling around, dealind ~100k shadow dmg to nearby enemies every 5 sec.  The dmg received decreases the further away you are from him.
   1. Empowered Whirling Corruption (gained at 25 dark energy) - The whirling corruption will periodically fire a missle at a player, dealing ~200k shadow dmg to all players within 8 yards and spawning a Minion of Y'Shaarj at the location.
      1. Minion of Y'Shaarj - Melee mob, when defeated will cast Empowering Corruption, healing nearby Minions of Y'Shaarj to full health and increases their dmg dealt by 500%.  Focus and kill quickly.
2. Touch of Y'Shaarj - Will randomly select and mind-control players, who will try to infect and mind-control other players.  Reduce the players health to 20% to break them free.
   1. Empowered Touch of Y'Shaarj (gained at 50 dark energy) - Transforms the selected player into a Sha that is immune to all CC and has increased health.  Will still continue to try to spread the Touch of Y'Shaarj, bring them to 20% health to break them free.
3. Desecrate - Same as Phase 1, throws a Desecrated weapon at random players, dealing ~160k shadow dmg to anyone within 15 yards, creating a void zone.  Destroy the weapon to eliminate the void zone.
   1. Empowered Desecrate (gained at 75 dark energy) - When Garrosh throws this, the weapon is now invulnerable and cannot be destroyed!
4. Gripping Despair (Tank warning, require swapping) - Tank deals, 20k shadow dmg/1 sec for 20 sec. Stacks.
   1. Empowered Gripping Despair (gained at 100 dark energy) - When Gripping Despair expires or is removed, causes Explosive Despair.
      1. Explosive Despair - Deals Shadow dmg equal to the value of Gripping Despair (possible raid-wide AoE).  Increases the dmg taken by Explosive Despair by 5% for each stack of Empowered Gripping Despair when it either expired or was removed.
         1. I'm assuming this means that if the value of GP was 200k from 10 stacks of the debuff, then once it expires, it will explode for base 200k, plus 50% of that(5% x 10 stacks), so another 100k dmg = 300k total explosion.  The dungeon journals Ive read doesn't say that this is raid-wide, but I'm assuming it is.  Either that or its meant to be more powerful than I'm understanding to force tank deaths to cause wipes. 

Phase 3 - Once Garrosh reaches 10% health again, he will run to the middle of the room and siphon any remaining energy from the Heart and will go to full 100 Dark Energy, all his abilities WILL be empowered, and he will regain ~20% max health.

At this point there are no new abilities, all of his abilities are now empowered and should be dealt with accordingly.

Pop heroism and go to town, getting out of Empowered Desecrated Weapon void zones quickly, breaking people free of the Empowered Touch of Y'Shaarj, use raid-wide DEF CD's for the Empowered Whirling Corruptions, and pray you win.

TLDR/Summary

This fight isn't overly complicated for an end-expansion fight, but it is LONG and it hurts.  You can expect to have to be at the top of your game to get this kill the first few times. 

Phase 1 is all about burning down adds in the appropriate order.  Ideally Engineers->Wolf Riders->Warbringers all while bringing down Garrosh as quick as possible.  If possible, a strat could be to leave one engineer up to spawn one of the Iron Stars, as they can be used to kill adds, but its EXTREMELY dangerous and can still lead to easy wipes.  Stay out of Desecrated Weapons and kill them to eliminate the void zones.

Phase 2 is all about preventing Garrosh from gaining as much dark energy as he can so he cant empower his abilities too early.  When you get transported to the Realm just sprint and run, taking any adds with you til the end and AoE them all down to stop him from gaining energy.  Then avoid his frontal cone, and heal through the AoE.  Once in the normal realm, just deal with abilities appropriately by killing the Desecrated Weapons, breaking people out of the Touch of Y'Shaarj and using Def CDs for the  Whirling Corruptions. Rinse and repeat until 10%.

Phase 3 is just burn, repeat handling of more powerful abilities and pray you survive.