I never use Fatebringer, but as it us the only arc hand cannon that I posses, it gets pulled out of the Vault when the arc burn Nightfalls come around. Patience and Time is the sniper for this too. 18k per shot on the Devil Walker, teamed with Blackhammer, it does not take long. Same goes for Sepik. If things get hairy, you can always rely on Against all Odds, there is no finer machine gun in the game.