So, this is one of the few fights I didn't get a chance to do when it was current and I don't have the achievement. I'm willing to bet that since it came out as filler content a lot of other people missed it as well. So if anyone else is interested in getting this achievement, be online at 8pm server time Friday and I will try to make a group for it.
Yeah, Heroic Halion 25 was one I ended up using OpenRaid to get, so if you can't enough on Arthas, it's not a bad place to check for more.
I was able to get this on openraid, however there is rarely a 10m group going for it there. It is my last raid achievement (until they added in the mop ones) and its soooo frustrating!