They have a decent chance if they don't bunch up, but ...
... an intelligently played dragon has a lot of ways to defeat a party.
.. the characters aren't the ones from LMoP. I am planning to still start the new ones at level 1, but super fast track them so they are level 12 at the end
I'd normally suggest that you take your time and don't rush the campaign. This team needs to gel in order to take on what can be a tough opponent if you as the DM play the Gold Dragon intelligently. Maybe your players will thrive on the 'drink through a fire hose' style of campaign you have in mind.
If the dragon is encountered in the lair, then the lair actions, allies and legendary actions may be too much, or it could be a grand fight that they can win.
The players are also very new, we've only had 5 sessions of LMOP.
I see a nice mix of classes, wizard, rogue, cleric, paladin, fighter and ranger. As the campaign progresses, challenge them so that they have to play as a team. That right there, playing as a team, is what will allow their synergistic effects to take on a tough boss.
About that dragon ... when do they first meet this big boss monster?
You will note that an Adult Gold Dragon has the Change Shape ability. (MM p. 114) What provisions have you made for the party meeting the dragon in "other than dragon form" during the course of play? Also note that the adult gold dragon flies, swims (is amphibious), and can breathe underwater.
The dragon magically polymorphs into a humanoid or beast that has
a challenge rating no higher than its own, or back into its true
form. It reverts to its true form if it dies. Any equipment it is
wearing or carrying is absorbed or borne by the new form (the dragon’s
choice). In a new form, the dragon retains its alignment, hit points,
Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair
actions, and Intelligence, Wisdom, and Charisma scores, as well as
this action. Its statistics and capabilities are otherwise replaced
by those of the new form, except any class features or legendary
actions of that form.
If this gold dragon is a major presence in the campaign and is played intelligently, the Change Shape ability allows it to get around quite a bit in other forms: human, elf, donkey, elephant, horse, giant eagle, etc. It can interact with PCs and NPCs, as well as gather rumors and news which makes it likely to have heard about this party well ahead of time. This gold dragon could even hire the party for a mission (while in human form), appearing as a "quest giving NPC" who needs something done during an earlier time in this campaign.
Just a note about intelligence gathering and spying: the best intel and recon is collected when those being spied upon aren't aware of the eyes on them. Conversely, when people know that they are being watched, they are more careful about what they give away. Apply that understanding to the dragon (sizing up possible opponents) and find ways to teach your players to do that same thing.
If this dragon encounter is to be a big battle, don't shortchange your players. Make it play smart: Int=16, Wis=15; this dragon is no dummy.
- Make sure that the party is challenged so that they have to act as a team in the encounters leading up to the dragon.
- Play the dragon smart, tough, and as a survivor. It didn't get to be an adult gold dragon by being stupid. All of that stuff in its hoard came from somewhere, and someone, right? That dragon beat tough, smart opponents (and a few fools) along the way.
About "...if they don't bunch up."
Try to make sure they fight at least one dragon, or one enemy with a breath weapon, before this encounter so that they understand the lethality of a breath weapon on a group that is bunched up. Sometimes, learning by doing is the best way. @András makes a good point about preparation for a fight against a dragon:
... not even the party cleric has a 50% chance to save against the Frightful
Presence on the first try, and the fighter will likely spend the rest
of his life (9 rounds) frightened. A simple Calm Emotions spell could
take care of this. Potions of Resistance are equally important;
without one the wizard could be taken out by two Fire Breaths (even
if he succeeds on both saving throws, and not much chance for that).
Some key features make an Adult dragon tough ...
(3/Day). If the dragon fails a saving throw, it can choose to succeed
DC 21 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns,
ending the effect on itself on a success. If a creature’s saving
throw is successful or the effect ends for it, the creature is immune
to the dragon’s Frightful Presence for the next 24 hours.
Who has proficiency in Wisdom Saving Throws?
DC 21 Dexterity save or take 66 (12d10) fire damage (half on a successful save)
Who has proficiency in Dexterity Saves?
The Wing Attack (Legendary Action)
(2 Actions). Each creature within 10 feet of the dragon must succeed
on a DC 22 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning
damage and be knocked prone. The dragon can then fly up to half its
Who has proficiency in Dexterity saving throws? Some of the party may spend the entire fight getting up off the ground