Skills

Cleave - Deadly cleave in front of Kazzak, can crit and should at no circumstances be taken by melee DPS.

Thunderclap - Short range magic AoE, deals roughly 2400 nature damage and slows movement speed by 60% and attack speed by 30% for 10 seconds. Dispellable, but due to it only being a short duration and not extremely high damage there is no need to counter it.

Shadowbolt Volley - A long range volley of shadow bolts dealing about 2000 shadow damage, partially resistable.

Mark of Kazzak - A non-curable debuff cast on a random player with mana that drains the target of 5% of his total mana per second over 10 seconds. If there is no more mana left to drain, the afflicted player explodes, dealing massive, raid-wiping damage to all around them. Casters need to stay above 50% mana throughout the fight because of this spell.

Capture Soul - When a player gets killed by Doom Lord Kazzak, he will gain 150,000hp. If you are killed by the opposite faction he will not gain any hp. This ability does not affect pets and totems, which can be used freely.

Void Bolt - A bolt he may cast only on the player with aggro. Deals 4000-5000 shadow damage. Also reduces movement speed by 60%.

Enrage - After 60 seconds Kazzak will enrage and increase all his damage done by 30% for 10 seconds, after that he returns to normal. During these 10 seconds he will do roughly 13k shadow damage to the entire raid.

Raid Composition

When setting up your raid, consider that Doom Lord Kazzak has got about 1,1 million hp to take down in 60-70 seconds, so focusing on having more DPS classes instead of hybrids and healers will increase your chances of getting him down before enraging.

Raid Positioning

Green = Ranged DPS
Bright Blue = Healers
Yellow = Melee DPS
Pink = Main Tank
Dark Blue = Doom Lord Kazzak

Encounter

The encounter itself is quite simple. Position ranged DPS (green) at range behind Kazzak, the healers (bright blue) up on the little hill almost where the Automaton trash-mob is, and melee DPS (yellow) is to be positioned behind Kazzak at all times to avoid getting hit by cleave. His hitbox is huge so you should make sure you are not standing inside of him.

He should be tanked with his back towards the raid to avoid cleave and the Main Tank should be standing in range and line of sight for healers.

The key to this encounter is to have as high DPS as you possibly can.

Remember: just because he enrages, it doesnt mean it's over. You can still take down at least 10% during his enrage or you can outheal it (not recommended).

Special Buffs

Shadow Protection - Reduces damage from Shadowbolt volley.

Hellfire Superiority - Extremely helpful buff and the most important one if you dont have a topped raid. Get the towers in Hellfire Peninsula and you will have an increased 5% damage for the entire fight. (tip: cap all towers to max and you will have more time to buff up)

DPS Potions - Melee and Ranged DPS classes need to maximize their output of damage on this boss, so any pots and flasks will help greatly. (Elixir of Major Agility, Elixir of Mongoose, Elixir of Major Strength, Fel Strength Potion, Arcane Elixir, Flask of Supreme Power, Flask of Relentless Assault, Adept's Elixir are all very useful)

Healthstones - Very important for the last part of the encounter, if/when he goes enrage you will want to use these to help keeping everyone up. Have your warlocks put up multiple soulwells while buffing up.

Party Roles

Main Tank
Take Kazzak and turn him around into position, build high aggro FAST, you want to start dealing damage to him as soon as he is in position. Getting 3 sunders up before nuking the boss will be enough, but you do not want to slow down; keep building more threat fast as the fight goes on.
The pull is simple. Don't use your bow/gun, instead run up to him (his aggro range is huge) and sunder him up while turning him around at the position marked on the picture.

Melee DPS
DPS Kazzak as soon as the tank is sure he got steady agro and push as much as possible, dump your agro if you feel you are about to get too much of it (warriors back out, rogues vanish)

Ranged DPS
DPS Kazzak as soon as the tank is sure he got steady agro, same as melee try to maximize DPS without getting Kazzak on you. (mages invisibility, hunters feign death, warlocks soul shatter)
One hunter use misdirect on the Main Tank at the pull and nuke to help him build more aggro fast.

Healers
Keep the Main Tank up, he will take heavy damage, but also make sure you can keep the entire raid up, one death and he will be healed for 150,000hp and it will be as good as over.
7-8 Healers focusing mainly on the tank should be enough and the rest healing the entire raid, if nobody in the raid needs a heal then you might as well throw a heal on the Main Tank.