The Pinger is the 3-legged alien mech that gets dropped onto the map, usually around halfway through the match. You need 25 kills with it using any of its three attacks: Left Trigger is the Plasma Mortar which can be equated to using rockets, Right Trigger is the Plasma Cannon which is a machine gun style firing mode with reticle bloom, and finally you can click the Right Stick to use the Ping Attack which is a shockwave that will kill nearby enemies, although they have to be fairly close for you to kill them it is quite effective. It should also be noted that if an enemy jumps on top of the Pinger while you're in it quickly use the Ping Attack as that will kill the player trying to rip you out of the machine. You can check your progress in the Multiplayer menu by going to Player Profile>Service Record>scroll to the Pinger icon (5th from the left).
Tornad0s reports that if you quit out of a game any Pinger kills you get in that game will not be counted toward this achievement.
You can find the Pinger on the following:
GAMETYPES Deathmatch
Team Deathmatch
Crash Site/Spears
MAPSAirport
Brooklyn Bridge
Financial District
Museum
Coastline (DLC)
Crossing (DLC)
Will update solution with any new info regarding other gametypes/maps that allow a Pinger.