I hate to disagree on this one, but I do. I hit 450 the other day. It's not hard, it's annoying as hell, tedious, but not hard. If you notice in a recent TWAB, Bungie said the the fastest way to get Primes is to get A ranks as fast as possible. I ran Caldera for like a week straight, ~8 hours a day. I tracked all the drops and found that while I was getting A ranks, I was averaging a prime every ~12 runs. At ~6 minutes orbit to orbit, I was getting a Prime every ~ hour 15 or so. When I started doing this, I was just getting B+ ranks and my average was more like every 20 runs.