TFF
      +--------
TPXUF | PXXQTRG
       -JQLXR
        =====
        MTJURR
       -MXNYRG
        ======
         MTQHLG
        -MXNYRG
         ======
          MGXJH

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