When you are in the sky, hold down R1 And press x repeatedly. This will make Eragon shoot arrows at Durza. Repeat this until Durza shoots fireballs at you. Dodge them. When the fireballs have a pinkish touch to them, hold L1 and keep pressing X. This will shoot them back. If Durza ever gets really close to you, hold X and breath fire. Repeat this process until Durza is dead.