FXQG
      +---------
TQRHR | GQQFYPHW
       -UPMNM
        =====
        TGMUTP
       -TUWXRR
        ======
         TFRWNH
        -TTNGNJ
         ======
           HPJHW
          -RXQNW
           =====
            LGLJ

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