Ive read your other post and YES, this is not a trivial task at all!
Adding polys and vertices is VERY maths hungry, and will eat HUGE amounts of
processor time, slowing game response beyond acceptable limits.

But if you are just deleting polys and vertices thats easily managable in
real-time, but it will leave actual holes in the model/entity that may
screw up dynamic lighting effects of the rest of model that has been cut.

I would go for a more carefully designed model that has patches of polys that
can be "lowered" using directx commands so they appear as 'deep pits' rather
than 'holes', and lighting will be OK then. Thats the cleaner way to do it,
and it will still be fast enough to be used in real-time.