Gothik the Harvester only has 2,510,100 HP in 25 min and 836,700 in 10 man raid modes. This might not seem like much, but trust me, this boss is no easy take down.
For the first four minutes and thirty four seconds of this fight, Gothik the Harvester will be invulnerable to attacks.
During this time your raid group must concentrate on keeping down the additional mobs which will spawn for the first 3 minutes and 44 seconds.
When Gothik the Harvester comes down after the 4:34 of trash clearance Gothik has only a few minor abilities which you must contend with. Gothik will teleport, fortunately agro is kept through teleports, making life much easier for the tanks.
- Shadowbolt - Gothik the Harvester will cast a shadow bolt every single second causing 4500 to 5500 shadow damage (2880 to 3500 shadow damage in 10 man) on his primary (tank) target.
- Harvest Soul - Gothik casts Harvest Soul once every 15 seconds, this reduces the stats of the whole raid group by exactly 10%, and stacks 10 times.
Since crowd control on undead mobs is harder however, the DPS on teh living side must control the flow in which they release undead mobs. Some groups may find it easier to place more DPS on the undead side, to help reduce the adds as quickly as possible.
You will need to have at least one tank for the mobs on each side, you should also make sure you have the tanks ready to taunt Gothik when he becomes vulnerable (Who will first spawn on the living side, then teleport to the dead side) When Gothik the Harvester reaches 30% health the room will become undivided, allowing you to quickly take down the remaining health of this add-adorned boss.
For the full list of adds and their abilities check out this WoW Wiki article.
I will however summarize how you should kill these adds.
First your living side team should kill the Unrelenting Rider, then the Unrelenting Death Knight, then the Unrelenting Trainee in that order. However, your living team must keep a close eye on the undead team, especially in the beginning since the Unrelenting Rider will spawn both a Rider and a Horse when it respawns on the undead side. You can CC a lot of the living mobs, which can help keep the rate which you kill the living mobs manageable for the undead team.
On the undead side, you will be able to take down the mobs as they appear, be warned though, the spectral rider and the spectral horse cannot be CC'ed!
Be careful not to overwhelm the undead side, and keep up a fast pace of DPS, it should not take you long to reach Gothik the Harvester who is a simple tank and spank job, with the slight twist that he will teleport between the two teams. This means that both your tanks will need to keep on top of him as he switches between the rooms. When he reaches 30% health you will be able to get your entire DPS on him 100% of the time, as the middle gate will lift!