use the weapons or tactics of your enemy or opponent even if you find them distasteful
1998 - New Scientist - Many opponents of biotechnology might say that they are simply fighting fire with fire. After all, the biotechnology industry is not averse to misquoting people when it suits them.
fight like cat and dog :
(of two people) be continually arguing with one another.
1995 - Edward Toman - Dancing in Limbo - Her desertion of him hadn't come as a total surprise... for the pair of them had been fighting like cat and dog for the best part of a year.
fight a losing battle :
be fated to fail in your efforts
fight or flight :
the instinctive physiological response to a threatening situation which readies you either to resist violently or to run away
fight shy of :
be unwilling to undertake or become involved with
1992 - Farmers Guardian - Welsh companies often fight shy of dealing with the big multiples.