I still think we can take him.
Well... Now I'm metagaming heavily but...
If our illustrious DM is merging the dracolitch into the rise of the dragonqueen (which is about Tiamat right?), it means that something *potent* has occurred to Dretchroyaster. He's not just an "ordinary" dracolitch anymore... Dretchroyaster has been... elevated, somehow, to something greater. This will probably cause the cult of the dragon - who are obsessed about these things - to rally behind him with renewed strength and fanaticism, as well as lesser evil dragons.
Before we can face Dretchroyaster, we will need to perhaps quest for some ancient weapon/artifact/ritual/ally/mcguffin, and face many of his lesser allies.
ooooor something entirely different!

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.