I've managed to snag Prometheus, Coldheart and my most recent was merciless. First drop was about 3 weeks ago and I play alot (although not specifically grinding strikes until somewhat recently) To note, mine all came from heroic strikes and no nightfalls and I had played a ton of those working from the NF specific loot. According to d2 checklist I have about 275 ranks across my character plus i'm sitting on another 2000 tokens.