Board Thread:General Discussion/@comment-74.10.105.99-20130523200150/@comment-24.73.157.98-20130523202348

the higher the difficulty of the stage you are doing, the higher chance you have of them dropping their unique armor piece. I have found a nice balance between difficulty and drop rate when I run the second hardest (Honor Difficulty). I think it is something like 80% for Epic, 70% for Honor, 60% for Valor, and so on. There aren't any specific tricks to get your drop every time that I know of.