Without a doubt the best way to get this is with the BASE Jumper.
Doing this achievement without it is very difficult unless you are an experienced market gardener.
The BASE Jumper allows you to slowly float down with a parachute, giving you more time to prepare for a strike.
Using the Sticky Jumper will help tremendously too, but is not imperative.
https://wiki.teamfortress.com/wiki/B.A.S.E._JumperThe kill can be achieved with any Demoman melee weapon.