The Crimson Dragon is the fourth boss you will face and its element is Fire so it is weak against Water. You can face it in the Flaming Wildwood or at the Summoning Stone. If you beat it 10 times, you will get the achievement Crimson Knight. Equipping your Knights with water element Armors is recommended, and it is recomended to avoid using spirit element Armors.
Crimson Dragon Edit
Max health: 187
Drops: Crimson Scales
Summon cost: 7 Gems
If you can collect four of its scales you can craft Dragonflame Armor.