Sounds like a Pseudonatural Adult Green Dragon to me (the first age category where it'd be Huge; any older and you're toast whatever you do).
If you've already taken 100 HP off it, you're halfway there. Keep whacking, it'll go down eventually. The way this sounds, flight is not a problem (there was a door mentioned, I believe?). The dragon can do good damage, but unless your ACs are really bad so it can Power Attack like crazy, it's unlikely to one-shot any of you.
What's more important is the kind of resources you've got left. Have you taken significant damage? Are you confident you can keep up the fight for at least as long as it has been raging? Is your primary damage dealer still up and capable of going? Have you taken the brunt of the dragon's breath weapon yet (rather nasty at 12d6 acid in a cone)? What do your remaining spells look like - anything to buy a round, debuff the dragon so that it matters (blinding is good; its saves will be hard to overcome, though), heal and/or buff the damage dealer(s), and most importantly, get away if things start to go really sour?
Sadly, Being optimistic about this fight, the 100 points that we have done to it, is actually like 80 points really. The Druid (Me) has barely any spells left, and most of them had to be converted into healing spells, (thanks to the feat Spontaneous Healer) just to make sure that everyone is still standing. Our Battle Sorcerer is polymorphed into Hydra and for some reason cant seem to hit the dragon at all, and if he were to change back and try to hit it with some of its spells, he would die from the Hit point change. Our main hitter/damage dealer is barely standing, and everyone else is a little less than half health.
As to the AC problem, unfortunately we are extremely under-wealth in this campaign, and we dont have the best AC (Our Base AC's range from 13-18 for the party). We also Don't have any debuffs prepared or we can use, and we cant seem to get past its SR to do any decent kind of damage with spells.
Our last problem is this. While running can help us at first, and will save us for maybe a hour or two, since we are stick in the very bottom layers of this dragon's lair, that is apparently filled with flesh golems, and goblins. If we try to rest and recover ourselves to try and either leave or get out, we will most likely get ambushed by the dragon or the goblins, that work for him (Among them is a high level caster who was able to dispel our rope trick, which is where we tend to do most of our sleeping). So, the only thing that really for sure guarantees our safety is really killing this dragon, and that seems to be slowly sliding out of our grasp.
I hope this clears up any questions, or concerns as to what the situation is like a little bit more.
Please and thanks for any help!