what is the best way to get liberator achievement?

[Liquidsnakehpks-joa]

10,000 kills is a lot , i have played the game lot already and still around 3.5k only can anyone recommend a easy level where you can get a lot of kills and survive for a long time without loosing the level.

i would prefer a level where there are no flying enemies

[ImprovedPhoenix]

the third level is perfect (survival on normal). There are a lot of walkers, so you will get around 800-1000 kills till level 34 (the moment I failed every time)