Thing is you can change your sub class, without changing any of your weapons/exotics/mods and still walk through a vanguard strike with ya eyes closed. Literally the easiest pinnacle in the game.
Ya know it used to rely on all three players having the same subclass. So many trolls running different subclass or changing right at the end to mess you up. At least now its in your own hands, and its only 3 matches. Trivial.
Just change your subclass before the chest appears. Maybe while the animation of the boss dying or between the death of the boss and the final dialoge.