Well i been playing gunz for 2 years and i know every technique thats been explained..first of all DBFly is Double ButterFly Which You Slash 2 Times In The Air. Plus All The Swords Can Do It...
Process :
Jump, Slash Block, Slash Block And When You Do It Make Sure Your holding Down the "W" Button to Move forward.
BAM! You've Got DBFLY =)