Kill an enemy with the javelin from 80ft.
How to: These things deal insane damage, you'd be crazy not to at least try to start (almost) every fight with one, which naturally inclines one towards throwing them absurd distances anyway. The only caveat for this achievement is that you would want to arrive late to a fight to throw a javelin at somebody who could probably be finished off by your other, less ammo-limited weapons.
You might try Cara, as people tend to forget their surroundings when contesting the territories, which can sometimes be seen from a considerable distance. The Last Team Standing maps are large enough, if you can survive long enough to be one of the few players left, and happen to be on the other side of the map to everybody else, and still have a spare javelin.
Pinegrove's a good choice, too. Stick to the back of the map (with the high bridge) where there's fewer trees.