XGGX
      +---------
JQXQJ | UXTWXLFL
       -JQXQJ
        =====
        QLQMNL
       -QHPHUG
        ======
         QHGPJF
        -QHPHUG
         ======
           JYNTL
          -JQXQJ
           =====
            MHXG

base: 15
letters: FGHJLMNPQRTUWXY
difficulty: challenging
missing: R