JFL
      +--------
PGXRM | JNJPQML
       -JXGYN
        =====
         GQGRM
        -FRNLT
         =====
          RWFPL
         -RPHNL
          =====
           FXJU

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