Being shot in the head is different for one young woman who lives in the moment, since instead of dying, she receives the responsibility of keeping heaven and hell from falling apart, or else an evil far worse than anyone can imagine will destroy everything and everyone.

I would like to know how she got this responsibility?

I am still learning all the nuances of loglines myself, but I am sure this can be tightened. Wouldn't something like this say it also?:

After dying, a simple woman must prevent all out war between heaven and hell or see the destruction of everything she once knew.

Who:
A simple woman who happens to be dead.

Want:
Prevent the destruction of want she once knew.

Obstacle:
A brewing war between heaven and hell.

Hello, there is just one point that is not clear enough "she receives the responsibility of keeping heaven and hell from falling apart": I can't imagine anithing corresponding visually to this. What does she do concretely in the movie? Secondly I would cut "worse than anyone can imagine" to make it shorter withour loosing any information.