@crash_happy: I'm going to try this one tonight. One of my friends said he did this for an hour and got a bunch of blues. But he set the difficulty higher than the guy does in the video.
I've been doing this one a lot. I'd say on average I get about 10 blue engrams (and the occasional single purple engram) per 2000 kills. I also set the level to 20 to avoid accidentally killing the knight or acolytes.