This achievement can only be obtained on the map "Coastline." Once the map starts, run towards the docks and look for the wooden paddle boats. Once you find them, each boat should contain a wooden oar within. Press and hold "X," on the oar to pick it up and then simply get one kill with the oar.
I believe that throwing the oar (RT) will not count as a kill, so its best you sneak up behind someone and melee them with the oar by clicking in the right thumb stick.
If you are going to leave negative feedback, please post in the comments and explain what you thought was wrong with the solution.