It's not easy to say who has the upper hand. Archer has the advantage of experience and a more complete set of skills. But Shirou has his own tenacity and the ability to surprise. In different situations during the battle, either of them could seem to be in control for a while.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.
Well, it varies. In some stories, the Daleks' sheer power and ruthlessness give them the edge. But in others, human ingenuity, courage, and unexpected alliances can tip the scales in their favor.
A simple Nezuko and Zenitsu fanfiction could be that they go on a journey together. Along the way, they encounter other demon slayers who are initially skeptical of Nezuko. But Zenitsu stands up for her, explaining that she's different. This shows the depth of their friendship and how Zenitsu has come to trust Nezuko completely.