For this achievement you need to kill 10 enemies by melee while holding a shield, now the achievement doesn't have any match type requirements so you can work toward this while playing online in a ranked match or split screen with a friend.
With that in mind if you're looking to get this achievement out of the way the quickest way to get this achievement is to set up a local game, set the game type to King Of The Hill, picking this will mean that your have infinite respawns during the game, this way you have absolutely no threat to contend with as the second player won't do anything unless you make it.
I've always favoured Pavillion while going for this achievement, it's a small map, grab the shield from the top of the map, run to the middle of the map and then use your second controller to move your dummy player into place, when you get him to the middle of the map switch back to your controller, kill him by tapping "B" a few times, this is a melee kill, repeat this process until the achievement unlocks!