This achievement will not unlock using the shields in Spears mode, unfortunately. To get this achievement, you should play Standard Team Deathmatch. You'll then want to pick up some kind of throwable object, such as a pole or wall vent. Ideally, use cloak to sneak up on people, throwing the objects with . You can kill someone with the object, pick up the same object and then kill another person. You can also die in between as long as you go back and use the same object. You can also get the achievement if you manage two kills in 2 throw. This achievement is known to be a little glitchy.