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.