You will have to kill someone by performing an airstomp, which is done by jumping from a ledge and then holding . This will make you "stomp" the ground, killing anyone under you. The attack has a blast radius that will kill anyone near you.
The best idea for this is to go somewhere high, above a high traffic area and use cloak. Wait around until you can see running under you, jumping off and then using the stomp to kill them. Crash Site is an easy game mode for this, because enemy movement is very predictable, as most people will be trying to get points inside the crash site.