Keep your Mages away from Draconis? Hee, hee, hee... ...
Draconis should stay away from them...
it is not good for his health.
But seriously folks... Draconis's human form is a pushover, simply send either a Deva or a Planetar to smack him down until he decides to turn into a Dragon.
True Sight should be maintained at all times.
Lower Resistance is a must if you want to use magic, a Spell Trigger filled with 3 of these will do the trick.
Khelben's Warding Whips and Breach will remove all his spell protection hoo-hah.
Use standard anti-Dragon tactics, never stay as a group, attack from all sides, so that his Wing-Buffet will not affect everybody.
Always have summons to help, Mordenkainen's Sword is gold here.
Etc... ...