The Flash is one of the fastest men alive, but even he still has a weakness that can be exploited by non-powered people like Batman. There haven't been many times that Batman and Flash have fought each other, but in the rare times it has happened, Bruce Wayne has proven he has a few tricks up his sleeve that could be used to take down the scarlet speedster.
Batman and the Flash are usually good friends and one would think that due to the sheer speed difference between the two there isn't much Batman would be able to do against a speedster. But Batman has shown time and time again, that there are specific countermeasures one can take, even against the fastest man alive. Normally there wouldn't be any situation where the two would need to fight, but in cases like Reverse-Flash or the one-time Joker infected Flash with Joker-toxin, it's always good to have a contingency plan in place.
In Batman: Endgame by Scott Snyder and Greg Capullo, Batman has to defeat a Jokerized Flash and uses what he calls "Protocol Run Red" which coats the surface area around him with a frictionless material. Without friction, Barry can't keep a grip on where he's running, causing his speed to go out of control and crash into the nearest object. In Aquaman & The Flash: Voidsong #1 written by Jackson Lanzing with Collin Kelly and artwork by Jay Anacleto and Rain Beredo, Weather Wizard is able to detect when The Flash is getting close due to the air ionizing before his arrival. There is a multitude of scientific ways one can predict or deal with a speedster.
But what if Batman doesn't have access to his technology? After all, with how fast a speedster can be, would be very easy to take the Dark Knight by surprise. How does Batman hold up without any of his gear against a super fast person? Surprisingly well, actually. In Batman\Flash: The Button by Joshua Williams, Tom King, Howard Porter, and Jason Fabok, Bruce is attacked by surprise in the Batcave by Eobard Thawne, the Reverse-Flash. Batman is easily overpowered, unable to even land a hit due to Reverse-Flash phasing through any attacks, but Batman points out that if Reverse-Flash is standing on the physical ground, then not all of his body must be phasing. This allows Batman to stab Eobard through the foot, pinning him in place and letting him get a few good hits in before Barry arrives to help. In the pages of Batman: The Red Death by Joshua Williams and Carmine Di Giandomenico, readers see a Batman driven insane by the death of his family, using every single weapon from Flash's rogue's gallery, as he tries to hunt down and capture Barry, believing if he had his speed he could save everyone.
There are many examples, both in canon and in alternate universes that show regardless of if he has his equipment or not, Batman is still capable of at least landing some hits on the faster Metahumans around. As fast and as smart as The Flash or any other speedster may be, they do still have inherent weaknesses that can be exploited, just like every other hero.