Indeed, Black Flash was the one to defeat Savitar. The battle was full of action and unexpected twists, but in the end, Black Flash emerged victorious.
It's hard to give an exact time as it depends on the manga's plot development. But typically, Yuta Okkotsu shows up when certain events and character arcs lead to his introduction.
Since I'm not well - versed in Yuta Tanaka's light novels specifically, it's hard to say for sure. But in general, light novels often have simple yet engaging language, vivid characters, and unique story settings. His works might also follow these trends.