I'm pretty sure your a better Crucible player then I am , if you couldn't get this done I never will, I try like hell but me and Hand Cannon precision kills in the Crucible don't happen too often especially since they took my Better Devil's , just another Destiny gun I won't have , on the bright side I already do have the original one (D1) !!
I have a Y2 Better Devils with Range Finder and Timed Payload. Very crispy and does well in PvE.
My problem with the Ace of Spades in Gambit was that. I wanted it. I changed my whole demeanor. I was making bad decisions. If I just played Gambit just to play it like I do the Crucible. I probably could have done the quest step on my own. If I were in a fire team, that would have helped as well. That's why I'm waiting a little but I'll be going back into Gambit to just play the game mode. I think I'll enjoy the mode and get kills if I'm not "trying" so hard. I did it to myself.
What I would do, if I were you, is just play it. It will happen. It may not be in a hour. It may be over many many games, but it will happen.