I did it completely on accident.[spoiler]If that doesn't make me sound like a shit sniffer then I don't know what would.[/spoiler]But seriously, I was playing Crota's end, just trying to get the cloak for my hunter. I got all the way through, Crota dropped, and to my surprise, I got the flawless raider achievement. I thought for sure I had died at least once in the abyss, but I guess I didn't.