This question: How could Dumbledore ever become the owner of the Elder-Wand? just speculates on the transfer of ownership of the Elder Wand. But the Elder Wand is supposed to greatly enhance the powers of the wielder. Grindelwald knew he wasn't a match for Dumbledore on even terms:
Over the years, Grindelwald raised an army and began a reign of terror that spread through several European countries and murdered many wizards and Muggles, though he never attempted to take power in Britain for his fear of his former friend, Dumbledore, who was "a shade more skillful" than he was.
But the possession of the Elder Wand should have given him the edge to be able to defeat him. The Elder Wand is supposed to make a wizard nearly invincible, their spells should always be more powerful than the opposing wizard.
How was Dumbledore able to defeat Grindelwald when he was facing the most powerful wand ever made?