Another level that is great and perhaps the best in doing just turrets is Episode 3: Moscow - Chapter Battle of Nerves. This chapter has 4 turrets throughout the level.
Just do an offline campaign with bots on easy or normal. The chapter is quite easy and can be ran through pretty quickly. Do the chapter 4 times to get the achievement.
The last part of the chapter is also a great way to boost your weapon levels as there is plenty of different weapons to choose from at the end and endless zombies keep coming. Once you complete turning all valves on and it wants you to turn on the final one, dont and let the zombies keep coming until your desire.