Venom's superhuman strength, shapeshifting, and healing factor are too much for Batman without specific anti-symbiote preparation.
Batman has defeated superhuman opponents many times, but Venom presents a particularly difficult challenge. The symbiote grants Eddie Brock strength comparable to Spider-Man (roughly 10 tons), shapeshifting abilities, tendrils, and a healing factor. It also negates spider-sense, though this is irrelevant against Batman.
Venom's weaknesses to fire and sonic attacks give Batman a clear path to victory if he knows about them and has appropriate gadgets. Sonic Batarangs or incendiary devices could exploit these vulnerabilities. However, without prior knowledge of the symbiote's weaknesses, Batman's standard loadout may not include the right tools.
In a random encounter, Venom's physical superiority is overwhelming. The symbiote is stronger, faster in close quarters, and can attack from multiple angles with tendrils. Batman's armor would provide some protection, but Venom can simply rip it off. The symbiote's ability to mimic clothing and appearance also makes it difficult to track.
Batman's tactical genius and gadget arsenal include incendiary devices and sonic weapons that could exploit Venom's known weaknesses. His preparation abilities mean a second encounter would go very differently.
Venom's superhuman strength, shapeshifting, and healing factor give him a massive physical advantage. Without specific anti-symbiote preparation, Batman's standard arsenal lacks the tools to effectively combat the symbiote.
Venom wins a first encounter through raw physical superiority. Batman with preparation and knowledge of symbiote weaknesses could even the odds dramatically, but a random encounter favors Venom's overwhelming stats.
Batman also fights
Venom also fights