One off? So you mean, your party, there's the Dragon, awake and on top of a mountain of treasure, and---GO!?
Do you have any idea of kind of dragon? Age? Size? Situation? Anything like that?
Are you going to be able to talk about strategy with the party? Will there be any roleplay leading up to the encounter?
Scenario: We're a tight night group of 6 that have been friends for 20+ years. However, we're now spread out over a multi-state spread. We get together for events 2-3 ties a year but one is an annual guy's only week-long vacation (Fri to Fri) away from the stresses of the life and the families to hang out and game for the week. However, this year one of our group cant show up till day 3 and he'll be running the main game, so for day 1 and 2, two of the other guys are running connected 1-shots. The first will be a low-level (what we call a mudbeggery) game. Day two will be the same characters at level 10.
Effectively the premise is that when we were young adventurers we were protecting a town from a bugbear problem (day 1 mudbeggary). At the end of which we stumbled on an opening to a deeper cavern and stumbled on what appeared to be a small (well huge for us) abandoned dragon horde. We took it and proceeded to build up the town with it and built our own citadel. Now, years and bustling town later, the dragon has come back looking for it's horde.
That's pretty much all we know other than that the GM in question has a love of big bad monsters and it will effectively end up likely being a slug fest. My guess is it will likely be an Adult or even maybe Mature Adult if he really wants to make us sweat. What breed? No idea. He collects Monster Manuals (and to give a mental idea all Green cards in Magic) so it could be nearly anything but likely something obscure, rather than one of the common breeds.
There will likely be some roleplay and interaction with the dragon, but again knowing the GM, it will likely end in a slugfest. The situation of which I guess will be up to us. Do we defend our town with siege weapons? Do we try to go sneak up on it somewhere? Do we try to meet it on open ground away from the town? All that I am assuming will be part of the game so there is really no predicting.
It originally was simply stated: "Level 10 and you'll be facing a dragon."