If you're looking for a World of Warcraft Karazhan walkthrough for Attumen the Huntsman then you've found the best on the net.
Go into Karazhan through the front door and go to the left. Kill the undead and horses. In the stables at the end sits the undead horse Midnight. This is the first boss of Karazhan.
Attumen the Huntsman (and Midnight, his trusty steed) is perhaps one of, if not the easiest raid boss in all the World of Warcraft. You hopefully shouldn't have that much trouble with him. He was one of the few raid bosses in Burning Crusade that I remember us downing on our first attempt with no wipes. He's really rather simple and if you have the gear to progress in Karazhan, you should be able to down him in 2-3 attempts as he's just a gear check more than anything.
Make sure that everyone has proper consumables, including food, elixirs, health pots and mana pots. Deadly Boss Mods is also highly useful, and can be found online easily. Everyone should ideally have both Deadly Boss Mods and Omen (or another appropriate threatmeter) before they make their first journey into Karazhan.
Optimal Group Composition:
1 Warrior Tank (any epic'd tank is fine, too)
1 Offtank Tank (warrior, druid or paladin works)
3 Healers (doesn't matter)
5 DPS (doesn't matter)
Most of the raid should have level 70 blues if not a few crafted and PvP epics. A few greens are alright here and there, but don't let the guy in level 65 quest greens come.
Have your offtank to go in to tank Midnight. He should be tanked in the middle of the stables, and when the tank has sufficient aggro, DPS starts on the horse. When you beat Midnight down to 95%, his master Attumen will spawn. The main tank should grab Attumen as he spawns and tank him next to Midnight. A timely misdirect is useful here, if you have a good hunter in the raid. The Attumen tank should spam Disarm and Spell Reflection on Attumen to lower his DPS and reflect the curse Attumen casts. Druids and mages need to dispel this curse when they can. Melee classes need to avoid the front of Attumen because he uses a nasty Shadow Cleave.
DPS down Midnight until he gets to 25%, then Attumen will mount up. Aggro wipes when he transitions, so all DPS needs to halt for the main tank to pick up Attumen. He'll start using a charge ability, but he won't charge people who are closer than 8 yards away, so everyone needs to group up behind Attumen and right next to him, which both avoids his charge and nerfs his cleave. Once the main tank has good aggro on Attumen, start DPS again as normal until he drops dead. Congratulations, you've beaten your first boss in Karazhan!
If your raid is having trouble with Attumen the Huntsman, then you are having aggro problems and your DPS is dying when he switches, your tanks are dropping because they need better gear (tanks must have appropriate defense to be crit immune), or your tanks are dying because your healers are running out of mana. Make sure the raid has mana potions, health potions, and good buff foods. Also make sure that everyone has threatmeters installed and working and make them watch it! I'll soon be back with another guide on Moroes in Karazhan, then the rest of Karazhan, and after that gold and powerlevelling guides.