I actually got mine playing Action sack. I waited till I got a Stab-out game.(Team Slayer with swords) and I stuck together with the team, and waited till there were 2-3 opposing team players coming for me.
It might take a few tries, but there will be plenty of opportunities. Should not take more than 1 or two matches on Stab-out.
Just be careful that you dont get assassinated while going for the kills.
I hope this helps.