????????
      +---------
WTLVZ | PHHUUQGY
       -WTLVZ
        =====
        VQPNXQ
       -VGTVHY
        ======
         YXZNGG
        -XLHQZY
         ======
          WTJQHY
         -TUXRPY
          ======
          ??????

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