How to prevent freehub body damage?
From Stan's NoTubes FAQs on wheels and rims:


  Why is my cassette digging into my freehub body?  
  
  Stan's freehubs are made lightweight using aluminum like many other brands.
  We recommend using cassettes with a rigid alloy
  carrier (XTR, XT, XO, etc. - Figure 1) for the largest sprockets.
  Cassettes with individual cogs (Figure 2) may mark the
  alloy freehub, though marks are only cosmetic and will not affect the
  performance of the hub.


From your picture it appears the freehub takes the hit from the individual sprocket. 
Need the cassette to act as a unit and spread the load across all sprockets. 
Tighten down more or look for cassette that has more of a body across the sprockets (rigid alloy carrier).

See how the rigid body (left) shares the force across not just the sprockets but also between the sprockets.