I love steampunk novel and graphic novel. Personally I am not a fan of fantasy (too many repetitive quest going nowhere with a final deus ex machina to resolve the inextricable threads), but I can see that you mix both steampunk and fantasy genre.
The good thing about the logline is that the genre and setting is well defined. Now the bad.
All the rest. We know nothing.
Who is the hero? Instead of newly recruited soldier use a novice soldier.
Why must learn to face his demons? What kind of demons does he has? What has happened in the past to make feel that way? Grief-stricken, feeling guilty, feeling abandoned ...?
When does he mature into a mercenary? Before the quest, during the quest? If it is before the quest, the hero starts not a novice, but a cynical mercenary. If it is during the quest, then we have a transformation/maturation during the movie and we don't need to know that. It's a given that a novice soldier will become an experienced soldier later. In every quest you get better or you quit/die.
What does the hero want to accomplished? What is his goal?
Who is trying to stop him? BTW, as a rule unless the character is an historical figure logline don't contain names.
How can seven swords of sins threaten humanity? Is it a special weapons? Replace that by a mythical ultimate weapon.