A T-Rex is just a giant chicken. Unlike what Jurassic park would have you believe, they are just animals. Meanwhile our ancestors did hunt things like wooly mammoths which were about as massive.
A wooly mammoth clocked in at around 4.5 to 6 tons, while modern estimates for average T-rex is between 5 and 8 tons. So, the T-rex was probably a bit bigger.
Obviously no human has ever fought a T-Rex, but we are quite efficient at killing pretty much every animal we've ever encountered. I see no reason a T-Rex would be any different.
Note that no single human likely won out against a healthy adult mammoth either. Mammoths were hunted by larger groups of humans who planned and equipped specifically for that activity.
To specifically return to the original point about stone age weapons and killing megafauna - the most likely weapon-based way to kill mammoths was with grounded pikes, which you drive the animal onto, rather than by any weapons held in human hands. And that's fine if it is hunting season, and you and two dozen of your closest friends have spent a couple days along the migration route of the animals preparing a site.
But, if the animal comes along a foursome of folks with pointy sticks wandering through the woods? You probably aren't winning that fight. If the animal can also fly and breathe fire or somesuch... there's a reason why they are in a fantasy game, y'all.