Personally, I think that the skill Feast of Corruption is the one most difficult to get. I took a friend up there today - there being Dreadnought's Drift, and if you have never heard of that place the reason is that it is in the middle of nowhere - it literally takes forever due to the location of the closest outpost and the number of enemies on your way. So we travelled for an hour or two, and I got to laugh like crazy because the guy didn't show up.
(We figured out a way to tickle him awake though

Anyway, feel free to share your capturing experiences for the amusement of all ...
