Assuming you want to grind this after the campaign, the best place to do it is to 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 and the Incineration Cannon. You should get a checkpoint either here or just through the portal.
During this next section, there are about 6 Knights, 1-2 Watchers and a bunch of crawlers. I found the following path to be the most efficient to take out the Knights.
Once through the portal, there's a Knight directly in front of you. Hit him with the Hammer. Then run up the ramp to the right and look off in the distance down a few platforms. You'll see a Watcher move up from a Knight. If you can shoot that Knight fast enough with the Cannon, it'll take both of them out and save you some work.
Jump down. There are two Knights on this platform, and about 3 on the one below that. Go crazy with the Hammer and take everyone down. Then, either when you kill everything or die, restart the checkpoint. Rinse, repeat.
This is also a great place to earn a few others as well, especially Chief, Smash if you don't have it already...you're almost guaranteed to get it without even trying. The medals will also pile up as you get multikills.