This can be gained in both single player and co-operative play, but is much easier in co-op. You can access survival mode from the main menu at any time. This achievement also becomes much easier once you’re characters have maxed out their abilities and both players have a good knowledge of the game mechanics and combos. This should take around 20-30 minutes. Here are a few tips:
Build up to and retain a high multiplier, this will gain points quicker giving you a better chance of survival.
Play co-operatively, this pretty much doubles your chance of gaining this achievement.
Use Lori’s abilities, cloning herself and confusing the enemies are some the most effective moves in the game. Remember she also has the ability to heal herself and can hide in the shadows.
If you are using Rick, picking up debris and using it as a weapon does almost twice as much damage as his fists, making him a much more useful tool.
Another option is to choose area 1 and use Lori. Hide in the shadows in either the top left or right corners and just keep pressing to throw knifes from a distance. If no one is around, use poison and it will affect everyone while also letting the enemies move in range for more knife kills. If you get into trouble move to the other corner, hide in the shadows and heal yourself. Repeat the whole process until you have the required score.