Gothik the Harvester

Couldn't think of a catchy line to get ya interested in readin'.. so.. well.. this one goes without. Yeah. I'm sorry. I know. I'll get one for the next boss..

Abilities

Shadow Bolt (Shadow) - Your Warlock standard spell. Just with a quick cast.
Harvest Soul (Debuff) - Reduce all stats by 10% for 1 min. Stacks up to 9 times. Starts when Gothik teleports into the room.

You'll also meet the following mobs in the encounter:

Unrelenting Trainee:

Death Plague (Nature) - Their melee attacks places a disease on you that can stack. Cleanse it quickly if you can.

Unrelenting Death Knight:

Shadow Mark (Physical) - Whirlwind damages nearby melee players.

Unrelenting Rider:

Unholy Aura (Shadow) - Hurts the group on the western side of the room for Shadow damage every 2 seconds.
Shadow Bolt Volley (Shadow) - Hurls missiles of dark magic, inflicting Shadow damage to nearby enemies.

Spectral Trainee:

Arcane Explosion (Arcane) - Sends out a blast wave of magic, inflicting Arcane damage to nearby enemies.

Spectral Death Knight:

Whirlwind (Physical) - Whirlwind damages nearby melee players.

Spectral Horse:

Stomp (Physical) - Damages and slows you if you stand in front of the horse.

Spectral Rider:

Unholy Aura (Shadow) - Hurts the group on the eastern side of the room for Shadow damage every 2 seconds.
Drain Life (Shadow) - Drains 12000 health from an enemy over 5 seconds. Can be dispelled.

Class requirements

You need a good bunch of dps. But this will be a given if you raid Naxxramas more often.

The pull

One of the tanks on the western side of the room will engage Gothik and thus start phase one of the encounter.

Positions and strategy

The room you stand in is divided by a large gate into 2 sides. As soon as you engage the boss, this gate will close and you won't be able to change sides until the gate opens up again.

Phase One:

Your raid force will be split into 2 groups with a balanced amount of dps, tanks and healers. One group will stay in the western side of the room, the other team will move over to the eastern side. In the western side of the room you will spot 3 plates on the ground. During phase one lots of Trainees, Death Knights and Riders spawn on those plates. These will be killed by the western group giving the riders the highest priority. Make sure, noone of them crosses the gate.

Now the interesting part. For every mob you kill on the western side one mob will spawn on the eastern side. You may notice this by a soul traveling from the corpse of a slain mob to the eastern side. The only exception here is the Rider. He will spawn again as a Spectrial Rider and a Spectral Horse. Again, you will kill the mobs that spawned on the eastern side giving the Rider the highest priority. Make sure noone of the Spectrals passes the gate.

Phase one will be over after 4 minutes. During this time you will have killed 35 Trainees, 14 Death Knights and 4 Riders. On one side of course.

Phase Two:

After 4 minutes, Gothik will teleport himself into the room making this a mere Tank & Spank encounter. He will teleport between the two sides continuously and your different tanks will need to taunt him once he teleported. After a while and lots of beating the middle gate will open up and Gothik will stop teleporting, making this even more of a cake walk.

Note:
The hard part in this fight is to stay alive the first 4 minutes and not get zerged.

Normal Difficulty

The tactic doesn't differ on normal difficulty.

Boss Kill: