So I just completed it on Hard, and didn't get an achievement pop. Anyone got any ideas?
After the first time I compelted it, the counter for how many missions I had completed was stuck on 19/20 for a while, so I think it may be related to that. Maybe it doesn't always work out what you've done or something. But I still got my achievement for completing it first time before that happened, and I've got no idea what I did to make it roll over in the end.