The method involves using a custom map. Additional spawn points and nuke usage (every 1 minute) are added there. Within 15 minutes you can farm about 50-60 thousand kills.

Download: GoogleDrive[drive.google.com]

Here 2 files.
The easiest way is just replace main.dat in root game directory.

But if you already have modified main.dat (for example you have already some other custom maps) you need to add mp_assault_experimental.txt into main.dat to folder data/levels with some archiver. I used WinRAR.

You can also create your own map or edit mine. There's a map modding guide somewhere around here.

1. Multiplayer Game -> Host Game -> Assault -> choose our custom map mp_assault_experimental_name.
2. Setup yourself as Defender.
3. Start and use Nuke on enemy spawn every XX:30 sec.

It counts with AI.
You can check progress in game Menu -> Achievements.
You must play out matches to the end regardless of winning or losing.

Important 1!
If you suddenly want to modify map and add new spawn points (to make it faster), then expect more load and longer loading.

Important 2!
I would also not recommend going AFK for 15 minutes, since a large number of enemy units will load the game (or your computer hehe). It is better to work with Nuke every minute.