Search public games, (preferably most active torments) keep looking until you find a group in a rift that is complete.
If the group is done and they're just running around killing useless shit, turn that bitch in.
West side A-Town stomp their ass and pce out that yolo baggins group.
All trolling aside, probably the easiest and probably able to be botted as there is virtually no walking needed or chance to die.
Currently doing it and joining rifts that are close to be finished now I'm getting decent legs/hour killing boss and shards. Quickest shards ever.
Might piss people off tho but you need to level right?