The question of whether Batman Beyond can defeat One Punch Man is a fun speculative matchup, but it ultimately comes down to the nature of both characters and their respective universes.
Batman Beyond (Terry McGinnis): Terry is a futuristic version of Batman, equipped with advanced technology, combat skills, and the intelligence inherited from Bruce Wayne. His suit gives him enhanced abilities, such as flight, strength augmentation, and an array of gadgets that can help him in combat.
One Punch Man (Saitama): Saitama is a parody of superhero tropes and is known for his overwhelming strength and ability to defeat any opponent with a single punch. His character is designed to be virtually unbeatable, with his only struggle being the search for a challenge.
In a direct confrontation, One Punch Man's premise is that he can defeat any opponent effortlessly, which makes it almost impossible for characters like Batman or Batman Beyond, regardless of their intelligence and gadgets, to win in a straightforward battle.
While Batman Beyond might be able to use strategy and technology to level the playing field in a creative scenario, the inherent joke of One Punch Man's character means he would likely overcome any tactics employed by Terry. Therefore, in a typical matchup, One Punch Man would emerge victorious.