I guess since Blizzard "fixed" the Malthael fight it's quite hard to farm in efficient way.
You need to run through fortress and it can take some time with dungeon exploration tag.
Well I found the simplest way to do it, set your trinity settings to max pack size or ignore trash mobs, use a dh with danettas double crossbow setup for infinite vault, and you get to malth pretty quick! ;] You can thank me later lol.