A dragon that knows the party is coming should be able to wipe that party (or at least cause significant levels of damage) before that party is even aware they are under attack or manages to get close enough to threaten the dragon.
Party is camped, preparing to invade the lair the next day. Party knows the dragon is a red (let's just go with that) and has brought all sorts of anti-fire protection.
Middle of the night, the dragon grabs a pair of huge rocks and does a flyover of the camp, gliding silently, dropping the rocks on whoever it can hit. Then does a flyby pass breathing fire at everyone before flying back to it's lair...burning down tents, supplies, etc. You don't sleep while wearing your backpack, so the wizard's pack full of scrolls might count as an unattended object...and go up in flames. You can't wear armor while sleeping w/o a feat, so no feat? Fighter's uber-strong full plate of badassery just got melted. Rest disrupted for any spellcasters, delaying the invasion of the lair.
Dragon summons a host of minions to it, and orders them to invade the camp while the party sleeps at random times, disrupting all of their rest all night long.
Party finally manages to get into the lair. They're prepared for fire, but first they have to navigate the maze of tunnels leading up to the lair...tunnels filled with ice, acid, poison, electric...basically everything EXCEPT fire traps.