First of all, the guide isn´t from me, but i couldnt find it here.
All of the naturally immune mobs are fixed. Wintergrasp GY isn't as ideal for AFK leveling because the mobs actually die and it's pretty annoying to pull them up on the platform, as well as WG can simply not be owned by your faction when you feel like leveling Unarmed. Outdoor 1hp mobs / Violet Hold / whatever take too much effort and are simply annoying for classes without passive healing because they usually fight back (or they die and you have to find new ones constantly), and being out of town hitting something with bare hands is just, well, boring. There's only one simple & effective method left to do this without any effort, I present you the...
Torch of Retribution - Item - World of Warcraft
This weapon hits for 0. Nothing. Hitting with it, however, increases your Unarmed skill. See where this is going? You can just hit a critter in a city, still be online talking in your guild chat / trolling trade chat / whatever while getting your achievement. How convenient is that?
One thing to note, most critters in cities run away when you attack them. But, there is one which doesn't: the Sewer Frog found in Dalaran Sewers. Just find yourself a nice corner where most players don't run to avoid the pricks which will kill it, and off you go leveling (or alt ab).
Now, this most likely was posted about before (even though glancing at the results of "weapon skill" search, I only seen WG and other useless, annoying or fixed methods), but there are 2 things that I've never seen mentioned in discussions about unarmed leveling.
Intellect: a long forgotten/ignored function of intellect is that it increases the speed you level weapons with. That applies to unarmed too. A free bonus for casters, and something melee classes can get off AH if you're filthy rich and don't care (of intellect greens). The difference can actually be huge - a mage friend of mine got the whole thing in just 25 minutes, whereas most melee players with it whined about the last skill taking upwards 2 hours.
Haste: attacking faster, skilling faster. There's haste on most WotLK gear, and it's useful for both casters, melee and healers. You're likely to have some haste gear already. This especially applies to this method of leveling - on non-immortal mobs, it also raises your DPS (yeah, those 50 dps you're doing without a weapon), so getting haste has a drawback. When you're hitting for 0, it doesn't matter.