Thanks for responses so far.
First things first, yes the dragon in question is part of plot. He should be trapped somewhere under old old castle, because he once was causing destruction all around. Then he was trapped, because he couldn't be killed at the time (deal with devils). So evil side now tries to release him and good guys (ehm, well at least the neutral guys) are trying to stop them and the dragon. I plan the evil guys to actualy make the ritual to set the dragon free, but at the same time let the party to get their ritual done so they can actualy kill him.
Enough rambling though. I think I will send a weak white dragon (without magic) at them first, so they know what they are against (and use the dragon to make winter come faster and whatnot...). This way they should get some hold of tactics. I'm thinking of giving them nets and some NPC party support at this fight, with some time to prepare too. This way they could also get cloaks done from the wings for some flight against the red dragon latter on.
So, uh, if anyone has any idea it would be nice.
PS: Wrote the post in hurry, sorry for any mistakes.