Gothik the Harvester Tactics

"Gothik, the Harvester. A master of necromancy and conjuration, Gothik is said to be able to beckon forth legions of the undead at a moment's notice. It is with his guidance that even the weakest of Death Knights can raise the dead." - Commander Eligor Dawnbringer

Gothik the Harvester is a unique fight, in that it splits your raid in half and forces you to act as two 5-man groups for a majority of the fight. It's tough, but if you have a few hard hitters you can take care of Gothik with no problem.

THE SETUP
Gothik's room is divided into two halves. The "LIVING" Side, which is the side your raid comes in from, and an "UNDEAD" side, which is to the right of the living side. Before you engage, there is an open gate between them, but when you start combat, the gate will close and seperate the two sides from each other.
For 4 minutes and 34 seconds (4:34), the entire fight will be Adds summoned in waves by Gothik the Harvester. You won't get to actually engage Gothik until that time passes.
Waves will come out in a unique way for this fight, as only the Living side will be getting waves of living enemies. When these enemies die, however, stronger enemies are spawned on the Undead side.
What this means is, that for every enemy the Living side kills, a stronger enemy is sent to the Undead side.
Most of your heavy hitting members should be on the Undead side for this reason, but make sure you don't completely abandon the living side, as they can get overrun too. Splitting up heavy hitters evenly between sides is usually a good strategy.
when the timer hits 4:34 after engaged, Gothik will teleport down to one side of the gate and start attacking, applying a soft enrage debuff to everyone in the raid. Soon after, he'll teleport to the other side, and continue to teleport between the sides until the gate between them automatically opens and allows the raid to finish him off, winning the fight.
The Key to this fight is communication, making sure that your Living side knows when to speed up kills and when to slow down kills will make the fight vastly easier.

ABILITIES
PHASE 1: WAVES
Phase 1 is the most of the fight, so players should focus here. The following will be the adds that are summoned, their undead/living sides, and their abilities.

LIVING SIDE ADDS
Unrelenting Trainee - Total of 24 of those will spawn throughout the fight, always coming in pairs. They cast Death Plague, which does 85 (25 Player: 170) Nature damage per 3 seconds, stacking indefinitely. SPAWNS SPECTRAL TRAINEE ON UNDEAD SIDE.
These are throwaway trash mobs, that can and should be just blasted through.

Unrelenting Death Knight - Total of 7 of those will spawn throughout the fight. They cast Shadow Mark, which hits for ~3,500 (25 Player: ~5,000) on plate and applies a Shadow Mark debuff, which causes all Unrelenting Riders to be able to hit the debuffed players with shadow bolts.SPAWNS SPECTRAL DEATH KNIGHT ON UNDEAD SIDE.
These are slightly dangerous mobs as they allow the Unrelenting Riders to use their shadow bolt ability. If you can, try to get these out of the picture as fast as possible.

Unrelenting Rider - Total of 4 of those will spawn throughout the fight. Their Unholy Aura will hit everybody in line of sight for 350 (25 Player: 500) Shadow damage every 2 seconds. Shadow Bolt Volley will hit for 3,800 to 4,200 (25 Player: 5,700 to 6,300) Shadow damage, but will affect only people with the Mark of Shadow debuff. SPAWNS SPECTRAL RIDER AND SPECTRAL HORSE ON UNDEAD SIDE.
If you do not have anyone affected by the Mark of Shadow debuff, these mobs become fairly simple to deal with. These should be prioritized if you have the marks up, but otherwise the Unrelenting Death Knights are more of a threat. Also, take note that killing ONE of these will summon TWO enemies on the Undead side. Take care not to overwhelm the Undead side when killing this mob.

UNDEAD SIDE ADDS
Spectral Trainee - Trainees cast Arcane Explosion, which hits everybody in 20 yards range for 523 to 577 (25 Player: 665 to 735) Arcane damage. Spawned from Unrelenting Trainee.
These mobs can be killed very fast, and are just slightly stronger throaway trash in the fight.

Spectral Death Knight - Death Knights use Whirlwind, which hits for ~2,700 (25 Player: 4,000) on plate. Spawned from Unrelenting Death Knight.
These enemies can get painful for melee-heavy Undead side groups. If you have a lot of melee, these should be on a priority kill for the whirlwind.

Spectral Rider - Riders have Unholy Aura, which does 350 (25 Player: 500) Shadow damage every 2 seconds to everybody in line of sight. Drain Life drains 6,000 (25 Player: 12,000) hit points from the person with highest threat. They will use Unholy Frenzy on a friendly unit, increasing attack speed by 50% (25 Player: 100%). Spawned from Unrelenting Rider.

Spectral Horse - The Horses use Stomp, which hits for 2,313 to 2,687 Physical damage in front of it, slowing the movement speed of the affected players by 60%. Spawned from Unrelenting Rider.
This is what tends to murder Undead side players, as Living side killing one of their Riders will summon two enemies on Undead side. You need to prioritize killing Spectral Riders ASAP due to their high DPS abilities. Riders should always be killed first when spotted. The Spectral Horse is a pain due to a moderate amount of damage and a slow, but can be ignored until more conveniant to handle it.

PHASE 2: GOTHIK ATTACKS
This is when Gothik will teleport down and start jumping between the two sides and attacking.

ABILITIES
Shadow Bolt - Gothik will cast Shadow Bolts every 1 second to his primary aggro holder, hitting for 2,880 to 3,520 (25 Player: 4,500 to 5,500 ) Shadow damage.
Gothik's basic damage attack. it doesn't hit terribly hard, even when spammed, so healers will not have much trouble with it, especially if resistances for shadow damage are available.

Harvest Soul - Every 15 seconds Gothik will reduce the stats of the whole raid by 10%, stacking up to 9 times
This ability will continue to make the raid weaker and weaker as long as Phase 2 goes on. This means that prolonged fights with Gothik himself are punished. Try to kill him as fast as possible!

Good luck.