PWQG
      +---------
PGWXN | LXZUVGPH
       -GPJVX
        =====
        XUYMZG
       -XWHYWM
        ======
         XJNQPP
        -XYQHZN
         ======
          XWGHHH
         -XQYZTX
          ======
           PTMPG

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: challenging
missing: R