I thought it wasn’t just about power and speed?
These are superheroes we’re talking about. Any one of them has spent years beating every single villain who shows up. Heroes generically have an advantage in experience, and usually in strategy. Nightwing in particular led the second-strongest-and-most-successful team in the DC Universe for years and took down plenty of superpowered goons in his time. Deathstroke couldn’t even save one hostage without botching it, and then got his eye shot out worse than Ralphie from A Christmas Story. The guy is overconfident and has his own share of psychological issues. (And no depth perception, which is going to make any close-range fighting hell no matter how good you are.)
Villains are not invincible, and if there’s one useful skill heroes have, it’s repeatedly and consistently finding ways to beat villains. Deathstroke is a serious threat for Nightwing, but when it really counts, I think Dick could and would take him down.