Team Fortress 2: Where Eagles Dare
To unlock the achievement, you must be a Soldier, and you must achieve the highest rocket jump possible by pressing jump, crouch, and fire all at the same time. It's a bit annoying to figure out, as the criteria for the achievement aren't very specific, but once you know what to do it's simple as stew.
This one was really nasty for me, because I am a terrible rocket jumper. However, if you know a place with a high drop, I think rocket jumping right off the cliff will still let you get this one. At least, it did that for me. Same with Highland Fling, which is awarded for sticky jumping very far.
To keep this on-topic, I'll go over my next-most-recently unlocked achievement:
Team Fortress 2: For Whom the Shell Trolls
To get this, you have to bounce an opponent in the air with a rocket, then use the shotgun to off them. To do this, a friend and I played one-on-one. She was a Demoman using the Eyelander, I was the soldier (obviously). After several tries, we found that using the Cow Mangler does NOT work for this; you have to use a real rocket! This was corrected by using the Black Box (to help alleviate some splash damage). It became a bit difficult after a time and I thought about getting a Reserve Shooter to finish the job better, but then I managed to collect the achievement without too much issue right after.
I don't even know if the Reserve Shooter would have worked.
But I don't need the shotgun for anything else, so I'm back to Cow Mangler + Buff Banner/Concheror + Equalizer.