My family is from Germany, I don't know how to speak the language, although my grandfather was fluent. His favorite phrase was "Made strong through the struggles" and I believe it went something like "stark durch die Kämpfe". Just looking for the correct translation.
"Was uns nicht umbringt, macht uns stärker" (What doesn't kill us only makes us stronger) would be a similar quote by Nietzsche.
If this is idiomatic, a saying in Germany is
Das Leben hat ihn hart gemacht
this doesn't refer to battles but to (difficult) life in general as the hardening factor, but it's the only idiom that comes to mind.