In my efforts to obtain my Sinister Squashling, I was lucky enough to get 7 runs against the Headless Horseman last night...however, not so lucky as to get my pet. Having tanked him 6 out of the 7 times, I did learn a few things to heed to make the fight a wee bit easier.
Summoning and Tanking Position
Before summoning, be sure to clear he trash along the stone pathway and on both sides of the mausoleum where the pumpkin shrine is located. When the Headless Horseman is summoned, stand on the stone pathway directly in front of the pumpkin shrine. Position your cursor a bit over the mound of dirt and as soon as he rises from the ground, click on him to target him. After he rises, he flies around in a circle and getting this initial target prevents you from having to wait until he lands and, thereby, causing delays in acquiring your mob. Since he engages the person who summoned him, this will help you be ready to pick him up the instant he lands. Also, have the person who summoned stand near you and to your left side so the Headless Horseman has to run through you to get to them.
When he lands, he will be in the grassy area to the right of the mausoleum. Do not charge in to him, let him come to you and even back up some as you do your initial threat generating damage so he's more in the middle of the stony path. This will help when his head separates from his body as it seems to have the annoying ability to move through the walls of the mausoleum and the surrounding fences. This can be quite aggravating when you're trying to get aggro and you have to run all over the place around fences and jump up and down off the steps of the mausoleum just to get to get to the head.
Once you've positioned him on the pathway, continue to build your threat by using Mangle when it's cool down is up, Lacerate, and Mauls (don't forget FFF and Demoralizing Roar!). If you and your healer are well geared, it helps to Enrage so you can keep the rage flowing and generate more threat.
Dude! Where's his head?
Continue to beat him down, but keep an eye on his health. Once you see him at 3k health, transfer your attention to his head. Once he's almost dead, that little bugger will pop off and end up about 30-40 yards away and can be easily 'lost' especially on the last phase when his Pumpkin Fiends are about. So, don't take your eyes off his head for any reason so you know what direction it travels when it disengages...otherwise, your group mates will wonder why you're just spinning around in circles as you make a desperate attempt to figure out where it went! It also helps to have a target macro:
/tar Head of the Horseman
Even with the target macro, keep an eye on that head so you know what direction is goes. Do what damage you can to the head, once it's at about 60% or the HH has healed to 100%, it will rejoin with it's body. Continue to tank and generate threat so DPS can burn him down and go through another phase getting him almost dead and his head popping off.
We say Fiends, he says Friends
On the third phase, continue to generate threat and allow DPS to burn down the Headless Horseman. At some point in this phase, he will toss out 4 pumpkins. Be sure to position yourself in the middle of these pumpkin as you continue to tank the Headless Horseman. After several seconds, the pumpkins will sprout Pumpkin Fiends. Have your Challenging Roar ability at the ready and use it once these Fiends sprout. Continue to tank him and use swipe judiciously so you give plenty of attention to his friends...errr...fiends and be sure to keep an eye on his health. Just like the other two phases, you want to keep an eye on his head when he's around 3k health. As soon as it pops off, go DPS him with all you have. You want to burn him down as quickly as possible. If you don't kill the head before the horseman reaches 100% health, you have to do phase 3 all over again...and well, there is such as thing as too many friends...errr...fiends.
All in all, it's a pretty simple fight. The main keys to keep it from being frustrating is positioning him so you minimize his ability to move through fences and walls causing you to chase him down and keeping an eye on his head when it pops off so you know what direction it lands.
I'd also like to thank all my guildies who helped me last night. They are all very sweet and quite enduring of my pet obsession...they even keep and bring the pets I've given them to all the raids and some are well trained enough to have them out even when I'm not in attendance. ;)