Since the addition of Spartan Ops, there's a much less boring way I have found to do this. Load up Episode 5 (Memento Mori) Chapter 1 (Spartan Miller). At the start you'll need to run tot he top where you will be greeted by 3 waves of watchers, first one solo, then 2 groups of 7.
If you're also after these achievements:
then continue the mission and you will swarmed by a shit tonne (no seriously, see for yourself, this is a justified usage for that phrase) of knights crawlers and watchers.
It's also far less tedious than grinding those campaign checkpoints as there's an unlimited supply of different human weaponry scattered around to customise your killing sprees with. Enjoy!