To my knowledge, no one has successfully solo killed Doomwalker legit, and I've always enjoyed new challenges, so I attempted to solo what used to take 20-40 people in TBC to kill.
This actually wasn't my first kill. My first kill was before Christmas when there were witnesses (Plastique from Death and Taxes) as well as screenshots taken. I didn't bother recording my first kill because I didn't think I could actually do it, but I managed it and thus recorded the kill this time around. By far the hardest part of the fight is his 20% frenzy.
Lightning Wrath - Strikes an enemy with a lightning bolt for 1700-2300 Nature damage that arcs to another nearby enemy. The spell affects up to 10 targets, inflicting greater Nature damage to each successive target.
Overrun - Random target charge, knock back, and aggro wipe.
Crush Armor - Reduces the target's armor by 10% for 30 seconds. Stacks up to 10 times.
Frenzy - At 20% health, all physical damage is doubled.
My armory (might not be in tank gear):
Talent spec I used: