The context is that Ronixis is a commander who hands Claude a pistol and tells him to use it in case of an attack.
I'm confused at exactly what そんなことでどうする？ means.
On one post そんなことで means "such a (little) thing".
Someone on another site wrote to me that: Kind of hard to explain since it's an idiom, the literal translation would be "what will you do when you are like that? " and it's a phrase used when you expect more from the person you are talking to.