When you first start multi-player on the game you will be asked to choose a specialist type and each of these specialists has their own weapon, for example the Ruin type has the gravity spikes, the Outrider has the Sparrow etc. What you need to do is get 3 kills in quick succession with these specialist weapon. I found doing this with the Sparrow on domination the easiest way as it has good long range and is also effective as an explosive.