When would the expression Fight Fire With Fire apply and what's the better expression?

You use it when you are in some sort of conflict and you decide to use the same means or tactics as your opponent.

By “better expression” do you mean one that more more aptly describes this sort of situation in general, one that more aptly describes the particular case in your link, or an expression that expresses a higher moral purpose?

Expressions that express a sentiment similar to “fight fire with fire” (not necessarily a better fit in this particular case)
“eye for an eye”
“what’s good for the goose is good for the gander”
“we’ll give as good as we get”

Expressions that express a higher moral purpose (you would use these if you choose not to act like your opponent or possibly not engage in conflict at all):
“two wrongs don’t make a right”
“eye for an eye leaves the whole world blind”
“we will not stoop to their level”

That’s a great answer @Zissou
By “what would be a better expression?” I meant since you never fire fire with fire and you never fight flood with water etc. A very child asks “How can I fight fire with fire?”.....

But people do fight fire with fire, @flo. It’s pretty standard. And that’s where the saying originated.

Hello?

Wildfire jumpers will often set controlled fires ahead of forest fires in order to remove the combustible materials that feed the forest fire in the hope that when the fire reaches the already burned line it will not be able to keep going so technically that is fighting fires with fire.

@rojo That would correspond with “Prevent Fire with Fire” though.

No, the fire is already burning out of control. So you’re fighting it.

I believe you’re confusing Smokey Bear’s “Only you can prevent forest fires.”