A strong sword is to deal as much damage to the souls as possible before they despawn in an attempt to get all 3.
MSI can be used but it is not wise to travel fiend/undead levels in any Mercurial set and not every player has access to the BK set.
The Ash of Agni and Voltaic Tempest are excellent bombs to deal with the Scarabs, not only can they deal damage but the status HIGHLIGHTS them on the screen. It becomes much easier to see scarabs in the distance when you can see them smoldering.
Your three strategy points are good. But "need" is a strong word. Most of the items you claim are needed are not truly needed.
For example, if you have MSI+4 or know the AI well, you can simply outrun/dodge the harvester.
Also, maybe you could explain how Ash of Agni or Voltaic Tempest gets used?