The last obstacle in the protagonist's journey/story.
Explanation:
Since there isn't much context to the story, I am assuming that "<em>The last rung on the ladder</em>" could either have a literal or figurative meaning. If it's a literal meaning, the character might actually be climbing a ladder and they're about to take the last step (the rung) to get to the top. However, it's more probable that the author means it in a figurative sense. I believe that "<em>The last rung</em>" is referring to a series of conflict in the protagonist's life, and the author refers to the ladder and its rungs as the conflicts.
<em>Protagonist's story</em> - the ladder (as a whole)
<em>Conflicts faced </em>- the rungs of the ladder (hence "<em>The last rung on the ladder</em>"