Zip-lining enemy killed in multiplayer.
To zip-line, a player must hold X while on it. Shooting a player off will not count, the player must die. They will most likely already have to be injured for this to happen.

Online private match boostable (2 players required)

Shoot the other player and have them start using the zip-line. Shoot them quickly while they are on it to kill them.

Note: Solarii Character Faction are the only ones who can zipline.