I've never used the hammer or the spin attack because they take too long, but I find using the Biggoron Sword much easier. Since your reflection only has the Master Sword, he can't counter you properly and most of the time you can slash right through his defense. The thrust is particularly helpful.

If you only have the Master Sword, don't Z target, crouch with R, and then stab him low. He can't defend that low, but this way can be risky if you can't manuever quickly enough...

If all else fails, you can try using Din's Fire repeatedly. If you hit him every time he'll die before you run out of magic power.

Hope that helps!