Duel-icious is a 15 point achievement, earnable in co-op mode only.
 The Method
The achievement is pretty simple - you only need to get another player playing with you, and 'knife' that player (press down on the right analog stick). Wait or ask the other person to melee you back. Then, the duel will begin. This is easily done with a good weapon and avoiding the other persons attacks, or merely asking the other person to let you win. Wait until the duel is over, and you've just got yourself 15 Gamerscore.