Post updated to reflect Mindeen's updated information.
The Pumpkin Headed Horseman spawns in a U shaped area in the center of it in the Loping plains.
Have you raid force wear as much hit point gear as possible, 7k+ at a minimum, 10k prefered.
Clear all the junk along the edge of the U so it is clear to pull along.
Setup the tank at the top edge of the U and the raid force max range from the tank down the side of the U.
The tank pulls using Tower of Stone back to top edge of the U, the AOE will hit at the start. Once the AOE goes off dps has 45 seconds to burn hard. When the next AOE is due the entire raid force must be max range from the tank and have the guard tower of stone/block again.
At this point the PHH will drop a pumpkin patch on the ground that makes targetting anything impossible. The tank now moves to where the raid is, and the raid moves down further along the U. Continue to do this until he is dead or you have run out of space along the U.
If you raid can do 40k+ dps then he will only drop one pumpkin patch.