Do Fist of Crota mission. Rinse and repeat after you kill the sword bearer. Each run should only take 2.5 mins from the point of launching a mission from the destination screen to jumping on your sparrow and killing the sword bearer. Complete on Heroic as research shows you have a higher drop rate. Use a shottie for a swift kill.
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https://www.bungie.net/en/Help/Article/13160