Beyond what Aimeryan said (which is essentially hit and run tactics to the max), 1 or 2 spells on the Dragon makes a huge difference. Plain an old invisibility is the perfect spell for a predator like a dragon. It has an hour long duration, which means its prey isn't going to see it coming until it's far too late. At a minimum this would mean no pre-buffing.

Also, the starting combat range feels off. The only way things start that close is if the Dragon is ambushing the group (or somehow got ambushed). If the Dragon isn't using something like invisibility it should be visible from a long way off. Which plays in to the hit and run tactics mentioned above.