Having done something very similar recently I already had a script.

    for f in */*/*
    do
      destdir=${f%/*}
      tvshow=${f%%/*}
      season=${destdir#*/}
      episode=${f##*/}
      # Get season number
      seasonnum=${season##* }

      dest=$(printf "%s/%s - S%02iE%s" "$destdir" "$tvshow" $seasonnum "$episode")
      echo "mv -- \"$f\" \"$dest\""
      # mv -- "$f" "$dest"
    done