For this achievement you will need to head into the rush gamemode of zombies and gain a score multiplier of 100.
EDIT : Have been told that you need to pass 100 for the achievement to pop
When inside this gamemode whenever you kill a zombie you will increase your score multiplier by 1 (example if you were on a x5 you would move to x6). Other ways to increase your multiplier are things such as collecting yellow symbols that appear on the ground and picking up nukes. Your multiplier goes down when you are either hit by a zombie or do not perform an action that rises your multiplier within a certain time (about 25 secs).
Some tips for not getting hit by zombies are buying every gun/perk that you see so you always have ammo and sometimes perks may save you from being hit, also using the box and pack. these are both free and can be used every 30 seconds. GET THE SHIELD, THIS IS THE MOST IMPORTANT ITEM TO GETTING THIS ACHIEVEMENT. Another thing is that barriers can be rebuild instantly if you press
, you can also constantly have traps on.
This took me a couple of matches to complete and you can also work on the other Rush achievements while going for this one. I did this on the map IX for anyone wondering.
Happy Achievement Hunting !