Open ground, advantage of movement, breath weapon, spells. I'd say that means the dragon is going to win. There is one thing that I suggest you look to correcting in the future:
Dragons have a lot to live for, especially the younger ones. Dragons live for so long that they must be careful about what fights they get into, and how long they stay in them. Any time I run a dragon against the party, as soon as he hits 50% hit points, he leaves... it's just not worth it to him to stick around and
maybe get some loot. It's like the old saying about fighter pilots: There are old pilots, and there are bold pilots, but there are no old, bold pilots.
The only time I would allow a dragon to fight to the death would be when it's cornered with no escape or it's a female protecting its eggs. Even if there is the possibility of its horde being taken over, it is too easy for a dragon to think: "I will let them think they've won now, and when they are complacent I will return, kill them, and reclaim my property, as well as theirs." Dragons should never, ever allow themselves to get killed. They're too smart.
That being said, here's a website you might like to look at to make dragons even more ferocious:
B.A.D.D.
This is
Bothered
About
Disposable
Dragons, and gives tips on how to make your dragons worthy of the name of the game.
Have fun, and make sure your mystic thurege knows what the heck red dragons are immune to. Heh heh.