This is very easy to do. In one of the early missions, you will get to jack an APC tank and, before reaching the Hive you need to destroy, you will have unlimited rocket launcher ammo. Simply go wild on the dashing crowds to obliterate them. You should be able to fire 2-3 rockets in 5 seconds, easily killing 50. Make sure you're also running some over at the same time to help with the kill count.
If you don't get it early on, just wait for the Rolling Thunder event with the Thermobolic Tank. You should have no problem getting it then.