WHEN FIGHTING DURZA ON THR LAST LVL FLY AROUND and use magic arrows on I'm constanly and when he fires his purple fire balls at ya use L1+X to deflect them to kill him faster.
When on the URGAL ATTCK! Mision make sure to use your fire breath to burn the oil feilds to do massive damage to the urgals and evil dragon rider kings troops(gallabroitrax) and try to tail whip or use magic arrows on large groups of enimies to send them flying.
As eragon use the set enemie on fire magic weneva you can to kill enemies faster(you unlock it on the durzas fortress lvl). P.S. I have completed the game on every difficul;ty .