According to the plot synopsis on Wikipedia:
As Bane is about to execute Batman with a shotgun, Selina crashes into the lobby on the Batpod and mortally wounds Bane by blasting him with the Pod's large cannons.
The ultimate way in which Bane is defeated might seem anti climatic but the defeat was certainly not undramatic. Retracing the events that led up to Bane's defeat show that he was indeed defeated after a lot of struggle on the hero's part. The long fight leading up to his defeat involves Batman damaging his life-support mask which results in Bane being in immense pain. This is achieved by his foe (Batman) who manages to overpower him despite having being heavily defeated by him in the past. Batman manages to face and defeat Bane after escaping an inescapable prison to which Bane himself had condemned him to.
Bane's real defeat was Batman being able to damage his life-support apparatus after which Bane could hardly fight back. Following that, the plot twist (involving Talia being revealed as working with Bane) brings out the new villain. Bane's "second" defeat thus would hold little significance since Talia is still at large and Gotham is still at the risk of annihilation at her hands. The main villain in the final few moments of the movie is thus Talia and not Bane and as such, a more dramatic defeat of Bane would have little significance.