You will have to play through the 10 different levels (accessed by hitting down in orer to reveal the other levels). This mode can be done online cooperative, split-screen, or by yourself. It's easier with a partner because you can revive eachother. The number of enemies increases by 5 if you do have a partner along with you.

There are a total of 20 waves in this game mode, but you are able to start from wave 15 on all maps and still get the achievement, leaving only 6 waves per map. Some of them do get pretty hard, so I will explain how to make it go by a lot easier.

On all of the maps there is usually a good vantage point where you can pick people off. Explore the maps to figure these out. Use silenced weapons and as many gadgets as you can. You can kill each wave, then die on purpose if you want to restock. Unlimited attemps! Should take you around 15 to 20 mins per level if you do it solo. Here is an example video explaining some things.