NQR
      +--------
NPNQQ | TTYMWWP
       -NPNQQ
        =====
         LFQTW
        -WMXUM
         =====
         NWJQGP
        -NXPFRM
         ======
          NQNGJ

base: 15
letters: FGHJLMNPQRTUWXY
difficulty: hard
missing: H