Load up Midnight, Rally Point Bravo. You'll fight through a couple of rooms and down a shaft. (These rooms are some great places to get Watcher kills.) Eventually you'll go through some portals into a room with a lot of Covenant weaponry.
Grab the Gravity Hammer.
During this next section, there are about 6 Knights, 1-2 Watchers and a bunch of crawlers. Go crazy with the Hammer and take everyone down. You're almost guaranteed to get is just by smashing everything in sight.
This is also a great place to earn a few others as well, especially the Crawler and Knight achievements. Grab the Incineration Cannon when you get the Hammer, kill everything, and reload the checkpoint. The medals will also pile up as you get multikills. You should be well on your way to these if you replay it enough (and haven't already gotten them):
Edit: Someone asked for a video of where I'm talking about. I don't have a capture card, but I found this walkthrough.
Rally Point Bravo is about 9:00 in (I think, I'm doing this from memory).
The room with the weapons is at 14:40. Pick up the Gravity Hammer, and the next room has a boatload of
Crawlers. If you run into the lower area, they'll all swarm you. Go crazy!