Yeah, it's a pain in the ass. The way I've gotten that ONE, was by being the only One alive in Templar (who was barely alive) and taking relic, taking down his shield and killing him with ghorn and then I died, but we all still got rewards. Maybe try to do Templar relic, cause it seems, even for swordbearer in crota, that they get the BEST rewards. Good luck finding that gun (it will change the way u play and make u wanna only use that gun