For some reason, arrows shot at the face will simply pass through the body. It has 10 HP, but may be hard to kill as it can attack from 16 blocks away, making it very hard to hit.
 Fireballs are easy to deflect,(be sure to note that you can send backfire charges with your bow too) and can kill a ghast if you hit the ghast with it. Don't count on it though, and still use your bow as your primary weapon.
 Collect them. Also, it is a good choice to build it out of cobblestone, as ghasts' explosions are incapable of destroying it.
Aim for the tentacles.
Either dodge the fireball or hit it to deflect it.
Seeing that ghasts can fly, you will need to build bridges to get to the drop (gunpowder and ghast tears) and experience.
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