The weapon must be a +6. Since Paizo has yet to tackle "epic" rules, you would need at least a +4 weapon with bane against whatever you are facing to overcome DR/epic.
You'd need a +6 enhancement bonus on the weapon. So, for the most part, you're not going to do it with a standard magic weapon. A +4 bane weapon of the appropriate type would be the most likely candidate for the job. Smite evil also should work. For most other characters it's effectively DR/plot device.
