You have 5 players, and at least one OP class(Magus), though by 15 most of those under Pathfinder will be pretty powerful.
So here's the CR math:
number of players(X) times party level(Y)Divided by estimated group size(4)
So, (5x15)/4=18.75. Personally, because of the huge power levels of Wizard anyway as well as Magus being fairly OP, I'd go up at least +2 on that, so for a
fair fight you're looking at roughly a CR 20/21. Pathfinder gives rules on what sort of adjustment should be used for different degrees of fights(
http://www.d20pfsrd.com/gamemastering) and an "Epic" fight is a +3 adjustment, so now you're looking at a CR24.
Now you've got to ask yourself. Are you players powerhouses? If they've got a "hit it until it dies" mentality, I'd make a safe bet that they are.
This is the stats for Pathfinder's biggest dragon:
http://www.d20pfsrd.com/bestiary/mo...s/dragon/metallic-gold/gold-dragon-great-wyrm
And it's still only a CR23.
However, with a REALLY powerful party, having only one baddie can get his beat up real quick. I would suggest adding some minions of some lesser sorts of dragons or something to help spread out the damage a bit. According to Pathfinder's CR math, two of the above dragon's is only a +2 to the CR number, which places you right around your goal CR for your party.
Also: remember to describtely call out that "A vortex of energy swirls around a small stone on the Dragon's neck as you attack, seeming to draw in the power of your assault while leaving the dragon unharmed."...followed by villainous dialogue topped with: BREATH WEAPON!
