Call of Duty: World at War

Whilst playing the Shi No Numa (Zombie Swamp) map if you are fortunate enough to receive an Insta-kill bonus (a floating golden skull), try to score 10 consecutive melee kills (that means pressing the Right Thumbstick in to lunge with a Knife).

To increase your chances of getting this achievement, get a powerful weapon that can take out a lot of zombies at once such as a Thompson or a Trench Gun and wait for a room to fill with zombies - you can get a friend/friends to help but as soon as an insta-kill drops - go around kniving as many zombies as you can - ask your friends to lay off from killing any for a second or two unless they get compromised.

If you hit it correctly, the zombie's head should explode. This shouldn't be too hard to get providing you get the Insta-kill early on in a round - I know in previous games it has a habit of giving it to you near the end of a round and there will be no zombies left to kill.

If you mess up you can either carry on in hope that you will get another chance or just restart.

(Note: Any player can pick up the Insta-Kill bonus because it will activate Insta-Kill for all players anyway)

the easiest way to get this is to play a game by yourself (from the main menu go SOLO > ZOMBIES > Shi No Numa as it is the easiest level to get this on (it makes it easier because your team mates will usually kill everything your trying to knife)). durring the first round only use your knife so you can get some extra points to either open the gate or the stairs, once you make it to round 3 kill the first 10 or so zombie and let them get through the windows. durring this round you will either get the nuke, or the skull for insta kill. when you see the skull appear DO NOT GRAB IT, instead wait for it to start flashing (while avoiding zombies, dont shoot them as you want to use your knife on them) once it starts flashing there SHOULD be enough zombies to go crazy on, it took me 3 games to get it right. be careful tho, the can still kill you.
hope this helps.

Early on in the game, buy a gun you are confident in headshotting with (steady aiming upclose with a thompson, mp40 etc works well) and go somewhere with a wide area like the bottom floor. Sit in the corner and wait for the room to flood with zombie's and then start picking them off until insta-kill is dropped. If you get one, go on a knifing spree. If you don't get one, try again next round or start again from the beginning if you feel the zombies are too hard.