GRU
      +--------
UQWWU | QTGYEUG
       -UQWWU
        =====
        UWEFGU
       -UUXRXW
        ======
         FXFPYG
        -FHEPFR
         ======
          UXMTF

base: 13
letters: EFGHMPQRTUWXY
difficulty: hard
missing: M