With the Fantastic Beasts movies pretty much cancelled now, at least for now, we will never know how the famous duel between Dumbledore and Grindelwald happened back in 1945. That however has not stopped fans from speculating and making theories.
Some theories say that the elder wand has a brother wand and some others say that Dumbledore took a vial of Felix Felicis before the duel. If someone has powerful as Grindelwald really was the master of the elder wand, he would be dangerous and almost impossible to defeat.
But what if he wasn't the true master of the elder wand. That's another theory I heard. In FB1 when MACUSA duelled Grindelwald, it was Tina who disarmed him. So according to the logic that if you disarm the master of the elder wand whether by a spell or by force, as shown by Draco when he was supposed to Kill Dumbledore in the astronomy tower, wouldn't that make Tina the master of the Elder Wand? Or maybe even Newt since he was the one who pretty much tied his arms up, making him unable to move? Maybe that was just some BS made up by Rowling to justify Harry being the master of the Elder Wand.