Attacks and Abilities
Attumen
Basic Melee: 1.6k - 2.1k on tank, 8k on cloth. He can, and should, be disarmed. Immune to taunt.
Shadow Cleave: Cleave attack for around 4k.
AoE Curse: Casts every 30 seconds a wide-area curse that reduces chance to hit with melee and ranged attacks and spells by 50%. Can be removed by mages and druids.
Mount Up: When he or Midnight is at 25% he will mount up.
Charge: After Attumen is mounted, he periodically does a charge. This does over 6k on cloth, and causes a knockdown.
Midnight
Basic Melee: Around 1.8 k damage
Attumen and Midnight are both immune to bleed effects.
Strategies
At the start, Midnight is alone. When she's at 95%, her master Attumen is summoned, right at Midnight's spot. When either reaches 25% health, Attumen mounts and gains the Charge ability.
Preparation
Clear the whole area, because all trash mobs will come when you pull Attumen, save for the five next to the blacksmith. Trash has a 25-minute respawn, so a little speed is required during the clearing phase. Respawns will not happen during the boss fight.
Tanking
One tank should start the encounter by pulling Midnight. When Attumen spawns, he should immediately be picked up by the other tank and drawn away so that Attumen's cleave doesn't hit the Midnight tank. Disarming Attumen will reduce his DPS by about 60%, at which point he'll be less damaging than Midnight. Also, a warrior can use Spell Reflection before Attumen applies the curse, which is triggered every thirty seconds.
Midnight should be tanked well away from the raid. Keep him in the center of the corral.
Mounting
When either horse or rider are at 25% health, Attumen will mount. Aggro will be wiped, and if he is below Midnight's health, Attumen will be healed up to the horse's percentage. The only difference after mounting is the additional charge ability. The charge hits the furthest person who has no aggro, so it is possible to put a plate / mail user at the back end of the raid.
Keep in mind that he can and sometimes will use his shadowcleave while he's walking back to the MT after a charge.
Since aggro is reset when he mounts, it's a good idea for the tank (if a warrior) to use Shield Wall at that time, to give the healers some slack. This will allow the warrior to build threat again, without requiring significant heals. Also, it is possible to avoid all damage by having the tank turn Attumen around, and having everyone in the raid stand directly behind him. This avoids both the charge and the damage caused by Shadow Cleave.
DPS
Midnight should be focused on first. It is possible to switch DPS to Attumen when Midnight gets down to about 30%, so that Attumen has low health when he mounts up. This shortens the length of the phase where he can charge, although it requires tanking Midnight for that much longer, so this is not a recommended tactic.
Healers
Healers should keep everybody's health topped off through the whole fight, especially in Phase 2, because the charge can hit anybody if the group is not properly oriented. Charge victims should bandage to lessen the load.
Curse
Mages and Druids should focus on removing the debuff from everyone.
An optional (but highly recommended) tactic is to cast Curse of Weakness on both Attumen the Huntsman and his mount, Midnight. You can only do so if you have a warlock in the raid. In fact, most of the bosses become easier with Curse of Weakness, allowing healers more time to heal their tanks. Curse of Weakness at its most updated form reduces attack power by 350, making this fight much, much easier.
See
http://www.wowwiki.com/Attumen_the_Huntsman for additional info.