"To break the back of the beast" means, literally, that you conquered the beast (breaking one's spinal is kind of a final thing for most Vertibrates). So, metaphorically it means: to be 'victorious' solving a large and/or difficult job (as if you battled a large animal).

this may mean taking a crowbar and killing the evil wolf who attacked you )

The "beast" is figurative, meaning a thing, a problem or a situation that must be brought under control.