There are currently 2 ways to easily grind this achievement. I have created 2 Videos to document this below
1) I have a pop file with Custom wave on decoy set at 16k melee scouts. Add your upgrades and leave this overnight to rack up 16k kills. Link to the file is in the description
2) Pop file that contains 10 waves of 10k melee scouts. Unfortunately there is an F4 prompt before each wave so this will require input between waves or an autohotkey script to mash F4. Links to files are in the description