What are some of the craziest/hardest to obtain achievements in game? Are there any that haven't been achieved yet, or are super rare?
I don't know the name, but there's an achievement that requires something like 5000 quest completions that I've gotta imagine is an infrequently completed achievement just because of the hours of gameplay required to do it.
All of the "recruits rewards" achievements are pretty rare. Even more so when you consider you can't earn them anymore I got "ding-a-ling" which was traveling by bell so many times... I know there is one for the spires and for the druid ring, as well. It's some high number of uses, which sounds easier than I think it is, and I rarely see those achievements. Swimming under the influence is another one I forget is available -- max out both swimming and alcohol tolerance.
Patchcraft destroyer: kill 10k patchcraft mobs during Bristlebane day - still my #1 out there. Limited time per year, limited mobs per zone (3-4 spots, also contested) and you get about 105/hour IF noone is grabbing them as well. Enjoy!
Deino-Ninja Complete Zavith'loa: The Lost Caverns [Heroic] without any members attracting the attention of a compsognathus. No idea if that one's ever been done. My group always got hordes of the little buggers.
I have the Deino-Ninja achievement. We had a little group of 3 players and worked on it for weeks to get a clean run. The one I was never able to get was: Jump the Shark Jump from the waterfall in Brokenskull Bay: Bilgewater Falls[Heroic] onto a Dreadtooth shark. I have no clue how many times I jumped and was killed by the shark but never found the sweet spot to get the achievement.