Dragons can breathe fire which makes the gryphon is disadvantaged. But the gryphon has a chance evade dragonfire and attack, especially if gryphons are hypothetically faster than dragons due to lighter weight. Its believable that multiple gryphon riders can take down a single dragon rider due to the dragon being overwhelmed from all sides, but how does gryphon rider defeat dragon rider in one to one combat? What would be the gryphon rider's method of attack? Would the gryphon need to attack the dragon or would the gryphon rider need ranged weapons? Maybe the dragon rider himself would be the target of attack?