Print a Wavy String Line-by-Line

MATL, 16 bytes

Zv3L)t?yn:)2$S}i

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Explanation

Consider inputs 5, 'programmingpuzzles'.

Zv     % Input, implicit: number n. Symmetric range
       % STACK: [1 2 3 4 5 4 3 2 1]
3L     % Push [1 -1+1j]. When used as an index, this means 1:end-1
       % STACK: [1 2 3 4 5 4 3 2 1], [1 -1+1j]
)      % Index. Removes last element
       % STACK: [1 2 3 4 5 4 3 2]
t      % Duplicate
       % STACK: [1 2 3 4 5 4 3 2], [1 2 3 4 5 4 3 2]
?      %   If non-empty and non-zero
       %   STACK: [1 2 3 4 5 4 3 2]
  y    %   Implict input: string s. Duplicate from below
       %   STACK: 'programmingpuzzles', [1 2 3 4 5 4 3 2], 'programmingpuzzles'
  n    %   Number of elements
       %   STACK: 'programmingpuzzles', [1 2 3 4 5 4 3 2], 18
  :    %   Range
       %   STACK: 'programmingpuzzles', [1 2 3 4 5 4 3 2], [1 2 3 ··· 17 18]
  )    %   Index modularly
       %   STACK: 'programmingpuzzles', [1 2 3 4 5 4 3 2 1 2 3 4 5 4 3 2 1 2]
  2$S  %   Two-input sort: stably sorts first input as given by the second
       %   STACK: 'piermnlsomgzgapzru'
}      % Else. This branch is entered when n=1. The stack contains an empty array
       %   STACK: []
  i    %   Take input
       %   STACK: [], [], 'programmingpuzzles'
       % End, implicit
       % Display stack, implicit. Empty arrays are not displayed

J, 54, 29, 27 26 bytes

-1 byte thanks to hoosierEE

([\:#@[$[:}:|@i:@<:@]) ::[

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R, 68 bytes

function(s,n)intToUtf8(unlist(split(utf8ToInt(s),-(n:(2.9-n)-1)^2)))

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• -10 bytes thanks to @Giuseppe
• -17 bytes because I was silly
• -9 bytes and n=1 case fixed thanks to @J.Doe
• -3 bytes thanks to @JayCe