I just did Little Rocket Man (along with One Free Bullet, Neighbourhood Watch and the Cache COllector) just a few days ago. Not necessarily hard, just a pain when you are driving.
Difficulty because of length of time you'd have to spend to get it, possibly either of the Seriously (and 2.0) achievements in Gears of War & Gears 2. I played Gears 1 for what must have been 7-8 months, most nights after coming home from school and easilly racked up tens of thousands, if not more, kills from public lobbies. However, I just couldn't be bothered going into lobbies that my friends couldn't join when they were online, so I we just played casual. It would have been boring and frustrating having to kill so many people when you couldn't talk with anyone you knew, or have the fun that could be so easilly had in public lobbies. Still, Gears is one of my favourite games none the less.
For sheer difficulty, I can't decide. Mile High Club was a pain to get, along with one or two other veteran achievements on COD4, but then again I went round to a friends house after getting mine, and got it him in only three tries.