As mentioned by Voile below, this beta kata is duplicate of approved 7 kyu kata; my exact solution unchanged passes here.

Author is inactive since 2017.

Duplicate of https://www.codewars.com/kata/last-man-standing

It's already fixed

Instuctions and sample cases added to the description. I also added example test cases and 100 random tests.

If you think this resolves your issue, please mark it resolved (and vote if you want to)!

Add instructions and sample cases. Beginners might have a hard time knowing what "flattening a list" means.

Done. Final testing takes values from random.randint().

Random test needed

Not really a grave issue, but you typed in

poly_derivative([1, 2] ) = [2]
poly_derivative([9, 1, 3]) =  [1, 9]

instead of

poly_derivative([1, 2] ) = [2]
poly_derivative([9, 1, 3]) =  [1, 6]

in your instructions.
I was confused at first (why is 9x the derivative of 3x2?!), but, yeah. Although test case resolves this.

Done.

The instruction is unclear

The example rendered as

123
123

123

instead of something like

123
123
123