The first part of the encounter he is polymorphed into human form. It would be interesting to have him be another type of humanoid for example elf, and then you could just give him Elven Accuracy or whatever other racial power you wanted him to emulate. You will want to start the encounter off with Tides of Despair so he has the aura available throughout the encounter. Once Syrantharax is bloodied he should transform into Dragon form.
Once there you should allow the players to attempt to dispel the aura either with dispel magic or some creative skill use. Hopefully by switching forms in the midst of combat Syrantharax will remain interesting throughout the entire encounter. However he should not fight to the death so if it gets to be a grind, simply have him start to try to negotiate with the players. If they seem averse to negotiation you can have him fly away to fight another day or simply surrender and beg for mercy if you don't want to bring him back later.
I hope you find a use for Syrantharax. If you have any thoughts on design or want to report how he worked in play feel free to leave a comment, or find me on Facebook or Twitter. Until next time remember, don't let your DM tell you no. It's always, "Yes, and...?"