not a tutorial but very impressive drawings
http://metalslug.ohayo-nippon.net/
look at the "art gallery" sections
http://metalslug.ohayo-nippon.net/artwork0.php
http://metalslug.ohayo-nippon.net/ms2-x_art.php
http://metalslug.ohayo-nippon.net/msart.php
http://metalslug.ohayo-nippon.net/metal ... ce_art.php