Whizzy's Solution
Achievement Difficulty - 1 / 5
Estimated Time Span from Fresh Start : 5 Hours - 6 Hours
As with most achievements, it is easiest to attempt this in an achievement server where there are idle players.
You need to find a map with a very high area to parachute off with the "Base Jumper"
I found a russian server with a map called "TF 2 Fun/ Achievement".
There may be others with high platforms.
Note that only this is measured in VERTICAL decent not HORIZONTAL movement.