I found it easiest to get "Swordsman", "Desperate Fragger", "Butcher", and "Backstabber" in a single game.
With a partner, set up a Deathmatch on Hoodlums Backyard, set the max players to 2, and the kill count to 100. Have the person getting the kills get shot until they have 10 health or less. Then, proceed to have them knife the other player in the back every time they spawn, after they pull out their knife.