Reversencode the given string
Jelly, 6 bytes
ṚŒHZUZ

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Explanation
Consider the string ABCDEFGHI:
ṚŒHZUZ
Ṛ      reverse -&gt; IHGFEDCBA
 ŒH    split into halves -&gt; IHGFE
                            DCBA
   Z   transpose -&gt; ID
                    HC
                    GB
                    FA
                    E
    U  reverse each row -&gt; DI
                           CH
                           BG
                           AF
                           E
     Z transpose -&gt; DCBAE
                    IHGF
       implicitly concatenate -&gt; DCBAEIHGF

I never thought the time would come when I'd outgolf Jonathan Allan in Jelly…