[Bruno]

Dragons as written are flying caster monsters that don't fit in small places, not ground bound brute monsters. Most flying wizardy-type things do poorly when you whack them over the head. When fighting physically, dragons should be doing skirmishing tactics: move and attacks where they swoop in, rake, and swoop out.

Oh, you put them in a dungeon with a low ceiling and narrow walls? The adventurers encountered it at a range of 10 yards when they turned a corner or kicked open a door and the dragon rushed them for some insane reason?

Yeah. Dead dragon.

If you want the dragon to be "tough" in claw-to-sword combat on the ground with no spellcasting and no use of the environment, make the following changes:
1) follow the note in DFRPG Monsters and call its claws and bite "weapons" as well as its horns - no trying to bite the knight and getting a parry in the face turning into a sword slash in the face.
2) Give it about five times the hit points. Major Wounds will be nigh on impossible to land, so it won't face knockdown/stunning. It can just wade through the PCs, Overrunning them or even Trampling them for free damage while dishing out multiple attacks per turn and soaking up hits left right and center.


But that means that if it actually also uses spells and takes wing to make run-and-hit attacks on the PCs, it's going to murderate them.