Sometimes there is a bug where missing zombine or something. This level I simply "cheat" to beat it as soon as possible, not worth the time hunt down the missing zombie.
Method 1: Use Cheat Engine
Kill the first zombie then first scan value "1", then kill another then scan again will value 2. Continue the process until you left with a few like 3 or 2 record. Then just modify the value to 100. There you go, mission exit unlocked.
Method 2: Die
I am not sure if this bug was fixed but on the previous version upon dying the "enemies routed/ enemy killed" number does not reset. Which means upon respawning, you just have to kill few more for the exit to be unlocked.
Trust me, I have watch youtube walkthrough and follow exactly but still exit not unlocked.