I’d go for a hunting spear. For centuries they have been used as a means for hunting dangerous animals such as boars and bears. The long shaft of the spear would be a way to keep the beast at a safe distance, while the wide blade pierces its innards and the crossbar would prevent it from sliding in too deep and getting me too close to the dangerous claws while the bear is still breathing.
That being said it would by no means be an easy task to accomplish - a bear is a formidable animal, strong, fast and agile despite its size. Basically a lone hunter’s only chance is to be nimble enough to not allow to become pray themselves.