How do you defeat the 2nd boss in ocarina of time?

There are two phases to this battle:

Phase 1

You have to strike along Ghirahim's right arm to inflict damage. Otherwise he will just grab your sWord. Keep doing this until phase 2 begins.

If he grabs your sWord, you can release it by moving it away from his arm.

If he takes your sWord, you can (must) get it back with either a shield bash, or by making him throw it at you (get away from him).

Phase 2

In Phase 2, he gets his own sWord and uses it to block yours similar to the Bokoblins (or whatever the red goblins are called). I found it much easier to hit him while he tried to charge me or just after he teleported.

He can also throw red daggers at you; if he does that, simply hold down Z, and press A to Dodge the attack.

(Fi FTW :) )