When you get to the first Ra'zac he will run away after you have took of half of his life. When you find him again, he will sneak up on Brom, Eragon will tell Brom to look out and Brom will run away. Saphira will fly down and attak the Ra'zac. When you start battleing again, the Ra'zac will dodge the attak. Try and make sure he is heading towards Saphira and when he is neer Saphira she will attak the Ra'zac. Keep doing that then he will die.