Karazhan Attumen the Huntsman Strategy

This boss in Karazhan is a relatively straight forward tank and spank encounter with two separate phases. Phase 1 begins with the huntsman's horse "Midnight" in the middle of the stables. Typically your OT will start the encounter by engaging Midnight and tanking him because Midnight is the weaker of the two entities in this fight. The melee DPS on the horse can really be positioned wherever they want to be, just make sure to stay well out of the range of the Huntesman once he spawns due to cleave effects. After Midnight reaches 95% life, her master Attumen the Huntsman spawns who is engaged and tanked by your MT.
Attumen should be tanked in the corner opposite of Midnight, but still within LoS for heals, to make sure that the melee and OT don't get hit by his potentially harmful cleave. At this point, you're still within phase one, and your goal is to get Midnight to 25%. Upon reaching this point, Attumen mounts his steed and should be tanked where Midnight was at location A. The MT now takes over the role of tanking the mounted Huntsman and not the OT is relegated to the job of DPS. To my knowledge, his cleave is a 360 degree circle around him, but we position our DPS at his rear out of pure habit and paranoia. From this point on, the fight is a straight "tank and spank" enounter, DPS the Huntsman down and enjoy the lewtz.

Positioning

MT : During phase 1 you should be located at or near point B on the map. Once phase 2 begins, switch to point A on the map in place of the OT.
OT : Remember, you're only tanking for phase 1, pull Midnight and tank on or near point A for the duration of this phase.
Melee DPS : For the entire course of phase 1 you should be positioned in such a way as to avoid being hit by the Huntsman's cleave. Midnight does nothing special and actual position in relation to him is non-issue. During phase 2 I'm 90% sure it really doesn't matter, but if you're paranoid like we are, simply stand behind the boss as indicated by the blue line above and all is well.
Healers/Ranged DPS : Should be positioned at or behind the pink line indicated in the diagram to make sure that you're in LoS and out of cleave range.

Tips and Things to Keep in Mind
Attumen the Huntsman will periodically put an AoE curse on the raid that reduces chance to hit by all types of abilities by 50%. Simply remove the curse with a mage or druid off of the MT/OT first and then choose your secondary priorities. He also has a random charge in which he chooses a ranged player to run to, sometimes it does some significant damage, sometimes it doesn't. Either way, heal the target if needed and he will run back to the tank.