to do whatever it takes; to use any means necessary; to stop at nothing
Set phrase describing a willingness to achieve a goal by any method, even if ruthless or unethical. Often used critically.
He will do whatever it takes for money.
That company is known for stopping at nothing to win the competition.
A set phrase composed of 手段 (means, method), を (object marker), and 選ばない (negative of 選ぶ, to choose). Literally 'does not choose means'.