First of all, the basis. You fight with 6 NPCs in 10 men, 10 in 25 men. Most of the champions have 2,400,000 health in the 25-man version; some have only 1,900,000. In the 10-man version most have 403,200 health and some have 322,600. Generally the "clothie" will have lower HP.
I'll steal the table for the names and classes from wowwiki:
The main difference from a normal fight is that CC will have Diminishing Return like in a PvP fight (hence 1 minute CCs will last 12 seconds, then 6, then 3, then be immune for 20 seconds). Also, all the NPCs got a 75% damage reduction against AoE effects.
The only difference in heroic is that they got much more HP, they are immune to Taunt effects, and they have a PvP trinket.
Now, let's find out the NPCs and their abilities, and the possible counters.
Note: The link of abilities is a random one, take the damage as indicative since it may vary between 10 and 25 men version.
Death Knight
Abilities:
- Chains of Ice
- Death Coil
- Death Grip
- Frost Strike
- Icebound Fortitude
- Icy Touch
- Strangulate
General informations:
He's dangerous only if allowed to get in melee of the clothies. He's not a high priority target, and his DoT should be dispelled asap. Generally you can ignore him until the end of the fight. He's vulnerable to Disarm.
Defensive mechanics: He will cast Icebound Fortitude quite soon when focused, making himself immune to stuns.
Offensive mechanics: Generally he will use Icy Touch on his current target, and occasionally Icy Chains. Sometimes he will Death Grip his target which in that case should get the fuck away as soon as possible. If rooted / slowed he'll generally use Icy Touch and Death Coil on someone nearby.
Druid (Balance)
Abilities:
- Barkskin
- Cyclone
- Entangling Roots
- Faerie Fire
- Force of Nature
- Insect Swarm
- Moonfire
- Starfire
- Wrath
General informations: Pretty annoying though not dangerous. Can be safely ignored for most of the fight. The treant he summons are not really dangerous either. His DoTs should be dispelled however to reduce raid damage.
Defensive mechanics: He will Barkskin if focused. He spends part of the time using Cyclone and Roots on random people, so you might want to use PvP trinket to get out of Cyclones.
Offensive mechanics: He'll generally DoT up 2/3 people, cast Force of Nature on someone, then go spammin CCs.
Druid (Restoration)
Abilities:
- Barkskin
- Lifebloom
- Nature's Grasp
- Nourish
- Regrowth
- Rejuvenation
- Thorns
- Tranquillity
General informations: HoTs HoTs HoTs. They must be dispelled as often as possible. He's a easy target to CC for a lock since he can alternate Banish and Fear to play with the DR. The Thorns he casts on the other NPCs must be dispelled, since it will hit the attackers for 500ish damage on each melee. He should be CCd most of the time.
Defensive mechanics: Will cast Barkskin if focused, and pretty much spam HoTs and Nourish constantly.
Offensive mechanics: None.
Hunter
Abilities:
- Aimed Shot
- Deterrence
- Disengage
- Explosive Shot
- Frost Trap
- Steady Shot
- Wing Clip
- Wyvern Sting
General informations: Damn backpedaling hunters. He has very high DPS, and since he cannot be kited, he should be either CCd or nuked down as one of the first targets. Also comes with a pet but the pet can usually be ignored.
Defensive mechanics: Will generaly pop Deterrence if engaged in melee, then possibly backpeda...er... Disengage. He will also often lay a Frost Trap on the ground.
Offensive mechanics: Generally picks a target, shoots 3-4 times to him (possibly with Aimed Shot or Explosive Shot), then picks another target, repeat. Will also shoot Wyvern Sting on a random target which differs from his current one.
Mage
Abilities:
- Arcane Barrage
- Arcane Blast
- Arcane Explosion
- Blink
- Counterspell
- Frost Nova
- Frostbolt
- Ice Block
- Polymorph
General informations: Annoying but not lethal NPC. Can be ignored most of the times. His DPS is not high and his CC generally gets broken by the random AoEs going on. Still recommending to dispel his Polymorph.
Defensive mechanics:He will Blink if stunned, and use Frost Nova if engaged in melee. Will also occasionally Polymorph a nearby target. If he gets low on HP he will use Ice Block which should be Mass dispelled.
Offensive mechanics: Will generally alternate through his offensive spells. His most annoying move is Counterspelling a random caster, which might or might not be a healer.
Paladin (Holy)
Abilities:
- Cleanse
- Divine Shield
- Flash of Light
- Hammer of Justice
- Hand of Freedom
- Hand of Protection
- Holy Light
- Holy Shock
General informations: Weak healer, extremely vulnerable to interrupts. Stick a couple of rogues on him with Wounding + Mind Numbing poison and you are pretty much sure he will not cast a single heal beside Holy Shock.
Defensive mechanics: Beside healing, he spams Cleanses, and a very valid way to keep him busy is having debuffs constantly spammed, he will lose most of his time Cleansing. He will use Hand of Freedom on allies which are rooted, and Hand of Protection on allies who gets low on HP; Hand of Protection should be Purged / Dispelled as soon as possible. He will Divine Shield self somewhere after getting below 50%: must be Mass dispelled in order to interrupt the following 90k Holy Light.
Offensive mechanics: Will occasionally Stun a nearby target but that's pretty much it.
Shaman (Enhancement)
Abilities:
- Earth Shock
- Heroism / Bloodlust
- Lava Lash
- Stormstrike
General informations: Very dangerous NPC, high DPS, capable of gibbing people with Windfury proc + Stormstrike. Recommended first target if CC isn't possible due to dispellers. The Heroism / Bloodlust must be dispelled (better Mass dispel) as soon as possible.
Defensive mechanics: Will drop random totems, generally buff ones (Strenght of Earth, Windfury) and defensive ones (Grounding, Tremor, Healing Stream) and occasionally Searing totem. All of them should be destroyed as soon as possible, expecially Grounding and Tremor since they interfer with CCing.
Offensive mechanics: Will pop Heroism / Bloodlust as soon as engaged, then just pick a target and attempt at raping him/her. Will also attempt to Earth Shock his current target.
Shaman (Restoration)
Abilities:
- Cleanse Spirit
- Earth Shield
- Earth Shock
- Heroism / Bloodlust
- Hex
- Lesser Healing Wave
- Riptide
General informations: Not much annoying, can generally be CCd, or else stick a couple interrupters on him and you should be done. His Heroism / Bloodlust should be dispelled or Massdispelled as soon as possible though. Dispel his Riptide / Earth Shield as well.
Defensive mechanics: Like the Enhancment shaman, will drop totems, which should be destroyed as soon as possible. He will drop Earth Shield on a random NPC and then alternate Riptide with LHW. Will occasionally Cleanse Spirit, but differently from the Paladin he doesn't spam it.
Offensive mechanics: Will pop Heroism / Bloodlust few seconds after engaged, and will occasionally Hex someone or Earth Shock a nearby player.
Paladin (Retribution)
Abilities:
- Avenging Wrath
- Crusader Strike
- Divine Storm
- Hammer of Justice
- Hand of Protection
- Judgement of Command
- Repentance
- Seal of Command
General informations: His Avenging Wrath must be dispelled as soon as possible. He's easily kitable and he's dangerous only if he manages to get in melee of some clothies. Vulnerable to Disarm.
Defensive mechanics: Will cast Hand of Protection on self or a ally which gets low on HP. Will also occasionally Repentance a random target.
Offensive mechanics: He will pretty much pick a target and try to kill it, he will generally wait until he gets in melee to pop Avenging Wrath. He will also stun on cooldown a nearby target.
Priest (Discipline)
Abilities:
- Dispel Magic
- Flash Heal
- Mana Burn
- Penance
- Power Word: Shield
- Psychic Scream
- Renew
General informations: The most annoying healer, generally the first one to die if CC isn't an option.
Defensive mechanics: Will pretty much spam Dispel Magic on anything which can be dangerous, then alternate between Shield, Renew, Penance and Flash Heal. Will also cast Psychic Scream if engaged in melee.
Offensive mechanics: If left unchecked and not focused he will occasionally Mana Burn, but generaly he's busy healing so it's rare.
Priest (Shadow)
Abilities:
- Dispel Magic
- Dispersion
- Mind Blast
- Mind Flay
- Psychic Scream
- Shadow Word: Pain
- Silence
- Vampiric Touch
General informations: Very annoying NPC but easy to counter if dispellers are on toes. His damage is not impressive if his DoTs are constantly dispelled.
Defensive mechanics: Will use Dispersion if focused, followed by Psychic Scream. Will also use Dispel Magic on allies very often, though rarely on self.
Offensive mechanics: He pretty much DoTs 3-4 people and then alternates Mind Flay and Mind Blast on random targets. Occasionally might Silence someone.
Rogue
Abilities:
- Blade Flurry
- Blind
- Cloak of Shadows
- Eviscerate
- Fan of Knives
- Hemorrage
- Shadowstep
General informations: Annoying and very dangerous for clothies, also very hard to kite due to Shadowstep / Cloak of skill...er...shadows. Recommended to keep CCd. He will not oneshot anyone but can be very dangerous due to Wounding Poison + Fan of Knives.
Defensive mechanics: Will Blind random people, and Cloak of Shadows if focused.
Offensive mechanics: If getting focused he will spam Fan of Knives and Blade Flurry. If not, he'll generally chase clothies (yes, he likes to do so like any good rogue) and attempt to Eviscerate them.
Warlock
Abilities:
- Corruption
- Curse of Agony
- Curse of Exhaustion
- Fear
- Hellfire
- Searing Pain
- Shadow Bolt
- Unstable Affliction
General informations: Dangerous, not cause of high damage, but cause of Unstable Affliction: UA on people will prevent dispelling, which can mean a lot of damage if the Shadow Priest or the Moonkin are around. Recommended to CC, if CC isn't available, get him down after the Enha Shaman / War / DK / first healer.
Defensive mechanics: Will occasionally Fear someone.
Offensive mechanics: Will basically cast his DoTs on 3-4 people and alternate Searing Pain and Shadowbolts. If there are too many targets close he will cast Hellfire, which hurts, hence move out of it.
Warrior
Abilities:
- Bladestorm
- Charge
- Disarm
- Intimidating Shout
- Mortal Strike
- Overpower
- Retailation
- Shattering Throw
- Sunder Armor
General informations: Dangerous with very high DPS and hard to kite due to Charge. Recommended to CC or kill first. Stay away from him if he Bladestorms, and melees should stun or run away when he uses Retailation.
Defensive mechanics: Will Disarm people occasionally and use Intimidating Shout and Retailation if focused.
Offensive mechanics: Will cast Bladestorm on cooldown, and Charge his target.
General tactic advice:
10 men: 10 men will generally figure 2 healers and 4 DPS. The setup you might be using will change a lot from group to group, so i will not assume you got every single debuff. What you WILL need is:
- A Mortal Strike effect: hence, an Arms Warrior, or Hunter, or Rogue. If you don't have any of those you will face troubles.
- A Heroism / Bloodlust: while not mandatory, will make zerging down the first NPC much easier.
- A offensive dispeller: with best being a shadow priest, anything could work, as long as you can dispel HoTs and Heroism / Bloodlust.
- A Rogue: ok, not necessary, but makes life much simpler with Wounding + Mind Numbing. If not available, you might want to use 2 interrupters for one healer.
Now, the tactic you will use will generally depend on the healers you got and the CCs you can use. The Paladin and Shaman are easy to control without CCs (with the appropriate interrupters). The Druid can generally be controlled by a Warlock alone. The priest is the weakest of the healers and should really be killed first. On the DPS, you might really want to zerg down the Enha Shaman first if you find him, independantly from the other 5 NPCs. He has the highest burst, drops down the totems which are big buffs to the NPCs, and also has Heroism / Bloodlust. The second target, or first if you don't have the Enha shaman, should be either the Disc Priest, the Arms Warrior, the Hunter or the Warlock. This will depend heavily on which kind of setup you got and find. I cannot sadly write strategies for any combo since it would take years.
The general advice is: kill the most dangerous one first while keeping healers busy, then go from there.
25 men: This is a bit different, you will fight 3 healers and 7 DPS. However, you will generally have multiple kinds of CCs available. The first target to kill should always be a healer, unless you get Paladin Shaman Druid, in which case you should CC 2 of them, keep the Paladin interrupted by rogues, and focus down the DPS. In other cases, zerging the Priest is usually the best thing to do, then priorizing Enha Shaman > Arms Warrior > Hunter > Warlock. Spam of dispels from the raid healers and dispels / purges from the DPS is key for this fight, also people must be quick to Mass dispel Heroism / Bloodlust, bubbles and whatever else comes in their way: as soon as 2-3 targets are dead, the fight is over. Assign DKs on kiting the most dangerous melees, and if you got a prot warrior, stick it either on a healer or on a caster: he will lock that NPC down for quite a long time. Prot paladins, honestly, change spec or play Prot healers: you are useless here: in Normal you may play a bit with taunts, but in Heroic, the only thing you got is a 30 sec CD silence. Don't waste your raid slot, either respec Retri / Holy, or if you don't have a second spec, wear holy gear and help dispelling and healing, as a Paladin you still got plenty of tools to use. Don't forget about Hand of Protection, Hand of Freedom and Hand of Sacrifice, those 3 buffs are extremely powerful. Divine Sacrifice if available is another very useful cooldown for the pull when the 10 NPCs are still alive.
(Prot) Paladin specific advices
Now a bit more specific part: this is a forum of Prot paladins and I assume you who are reading this guide are a Prot paladin as well.
In Normal difficulty, there's quite a lot you can do as Prot paladin: you can keep taunting melees off clothies and can interrupt casts with Avenger Shield. This however isn't possible in Heroic, so you should try already from the facerollable Normal to do something else.
As i said above, if you got a dual spec, use it, else play prot healer which works as well.
Without considering the spec you will use, you got many tools with you.
- Hammer of Justice: 6 seconds stun, use it wisely. It's good to use on a focused caster (expecially healers) at the start of the zerging, before rogues have alrady Kidney Shotted.
- Hand of Protection: Save that clothie! If you see a caster low on HP followed by a melee, HoP him!
- Hand of Sacrifice: Lesser version of Hand of Protection, may or may not save someone, but 30% damage off that priest might be enough to save him. Use it if HoP is on CD and someone is getting raped.
- Hand of Freedom: Mainly should be used to free people from Chains of Ice and Entangling Roots.
- Turn Evil: If there's the Felhunter, you can fear it.
- Divine Shield: Well, as much as Hand of Protection can save others, if you get focused, bubble, cause generally they WILL rape you if they gang on you.
- Cleanse: This is your best friend: no matter what's your spec, defensive dispelling is a very important part of this fight. As long as there arent Unstable Affliction going on (in that case dispel = oneshot), removing DoTs from people is very important.
- Seal of Justice: Unless you are retri, you should bounce around with this: will proc on Judgement if Holy and on HotR if Prot. Might not be that impressive but it's still a way to stop people from raping your clothies.
Depending on the spec, you might want to play in different ways.
- Retribution: Pop Avenging Wrath on the start so you will have bubble ready later on if they zerg you, and you will also help killing the first target faster. Ignore consecration and Divine Storm, they are hardly worth the GCD due to the 75% damage reduction. Use your Repentance wisely on either a unchecked healer or a melee chasing a caster.
- Holy: Spam spam spam, and stay away from the mobs! If a mob follows you, run for your life and stun it if it comes too close.
- Protection: If you are prot healing, remember you got Avenger Shield to silence people, you should use that on cooldown, and you should also stun mobs whenever possible to save someone else.
Uhm, this is all for now, if you have suggestions for anything else to add, shoot.