The Elder Wand is not in fact an unbeatable wand. It has been defeated many times throughout its bloody history. Dumbledore was a more powerful wizard, so he could defeat Grindelwald even though he had the Elder Wand.
The Elder Wand is simply incredibly powerful, not unbeatable. It can perform some otherwise impossible feats, such as repairing a broken wand, but a sufficiently skilled wizard can defeat the Elder Wand. Its likely that Albus outsmarted, rather than overpowered Grindelwald.