Alright, so I've found these mobs in northrend that are immune to all damage. You can level your weapon skills off of them, not get hurt at all, and never have to switch mobs (Due to the fact they can never die). On to the location of these mobs.
I'm going to steal the screenshot from kebbsta (Thanks!) as I can't seem to edit my own, paint uninstalled itself from my computer for some reason.
The mobs are just on the ledges above this area. They're named Iceskin Sentry, they have a buff which "freezes" them in stone. Enjoy getting your free achievement and weapon skills :P. I've searched and couldn't find anything, if anyone does find anything, I will delete this thread. Happy Skill Leveling!
Quick edit: I just realized this is more of a tip then an exploit, although I'm sure blizzard didn't intend you to be able to weapon skills on a mob. If a mod could move this over to guides, I'd appreciate it. (If it does need to be moved, anyways.)