You're best bet is to load a save (assuming you have one) prior to approaching the stronghold when the dragons attacked and killed the chief and hope the dragons don't attack again next time. Unfortunately, sometimes dragon attacks will happen at a given location no matter what. You might try not fast travelling directly to the stronghold next time too, sometimes fast traveling is more likely to trigger a dragon attack. Try walking from a nearby town (or location instead). Several of the orc strongholds are located near random encounter spawn points, which occasionally will cause a dragon attack. If you don't already have the latest patch (1.9) from xbox live, download that too, it was supposed to lower the chance/frequency of getting a dragon attack.