# [05AB1E], <s>19</s> 17 bytes

    áΣl}.γl}éεgyθ‚}JJ

[Try it online!][TIO-jr1ps06z]
or as a [Test Suite](https://tio.run/##yy9OTMpM/V9TVvn/8MJzi3Nq9c5tzqk9vPLc1vTKczseNcyq9fL6X2mvpKBrp6Bkr/PfO7MkO7FEISQxLz2fyyWxJDEpsThVISCxqCSzJDM/rxgoU5wNAA)

[05AB1E]: https://github.com/Adriandmen/05AB1E
[TIO-jr1ps06z]: https://tio.run/##ATsAxP9vc2FiaWX//8OhzqNsfS7Os2x9w6nOtWd5zrjigJp9Skr//0RhdGFiYXNlIFBhcnRpdGlvbnMgVGFzaw "05AB1E – Try It Online"

**Explanation**

    á                  # keep only letters in input
     Σl}               # sort by lower-case
        .γl}           # group by lower-case
            é          # sort by length (stable)
             ε    }    # map each to
              g        # its length
                 ‚     # paired with
               yθ      # the last letter
                   JJ  # join to string