The easiest way to do it is in endless mode.

The only problem here is that the boss is pretty tough. In the most maps you'll probably not be able to kill all bosses in a big spawn at once (not even with rocket launchers).

The good thing is, that the boss can only attack you from above. What you have to do now is create a map which gives you the possibility to kill the bosses pretty fast with many rocket launchers.
Next step is to give them a roof ;). if the complete area where your rockets stand has its "roof" the boss will not be able to attack you anymore!

You'll now only have to get to the "boss rounds". Approximately at wave 100 you'll only have bosses spawning and nothing else! When you're set up pretty good now you can leave your pc and do whatever you want ;)

Here some screenshots of the map i created to do that crappy 50k kills achievement:

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As you can see, the enemys have a very long way to the friendly spawn.
Many Rickys in the foreground near the enemy spawn
Even more snipers in the background.

The roof will protect them from the boss attack!

you can get this map ingame!

the map is called PARKERS

have fun with it!