To farm kills, get the minigun as soon as possible. This can be done easily by playing the Biohazard missions. Depending on the length and difficulty, you will receive gold bars as a reward. Forty five gold bards are required to purchase the minigun. Then, select a difficulty that you are comfortable with and begin farming kills in the first survival level "AlphaComplex" repeatedly. If done correctly, you can get about at least hundred kills per run, which lasts three minutes. This will also result in a large number of credits and gold bars, which will help you upgrade your minigun and other weapons. In survival mode, the zombies spawn faster the faster you kill them. They also always spawn outside your field of view. Make sure you run around and look behind every corner. By doing this you will turn away from other corners and give the zombies a chance to spawn there.