Make a map in forge with only 2 spawns, save it and start it in private with the lowest health and make sure you change the spawn settings to instant so you don't have to wait a couple of seconds each time you or your friend are dying after melee each other!
Also disable killcam!
If you have a second controller I recommend using it instead of a friend to make it faster.
Then it will only be you boosting it for a total of 250 melee kills instead of 500 for both of you
Happy easy grinding people