This one looks difficult!
As for heads, since I mostly work digital, if I have to do a challenging pose I'll just grab a CSP 3D head and place it in the canvas for a direct reference xD.
Physically... not sure, I guess I would set some "anchor" points and make sure some distances between these look right, for example eye in relation to ear, nose and chin... cheek bones in relation o the sword placement, etc, then apply basic forms over it and finally add the hard pencils or ink. I think as long as those main point placements are correct, it will usually work out.
Then again, I rarely do physical xD