The only difficult achievement was Aggramar, and that was mostly because we didn't have a second tank who knew how to tank swap during the combo while keeping the boss from moving. We made our DPS warrior learn to tank. It took us a few tries, but we got the tank to combo one of the adds twice, and got the achievement.
We also accidentally killed the last boss on Heroic. The achievement requires you to be in phase 4. So we unloaded on the boss in phase 1 to speed things up. Unfortunately we sped things up a little bit too much, and killed the boss as he was transitioning from P1 to P2. So we had to go to Normal difficulty and slowly whittle him down and make sure all the phases transitioned properly.
We did get all of the boss achievements this week, giving most of us the meta-achievement for that instance, and some sort of mount. Kind of interesting to go back to it after all this time.