Here is a way to do this achievement solo:
1. Get the whitelisted "FakedPlayers" mutator steamcommunity.com/sharedfiles/filedetails/?id=305337326
2. Go to "Host game" and choose any map and difficulty you want(beginner works)
3. Under "Mutators" tab enable the "FakedPlayers" mutator and under "Configure mutators" button, make sure "Forced Minimum Players" is set to 2.(doesn't work with 1)
4. With all the settings done, host a "Listen" server. Finish the first wave and it will count towards achievement.
After the trader timer starts, you could exit the server and host it again so you don't have to wait for the timer to run out. First wave also has way less zombies, so it goes much faster. Keep repeating until you get achievement.