There's only one boss you have to fight in the Crown of the Old Iron King DLC in Dark Souls 2, but he's a real dick. After descending Brume Tower, you come face to face with the dual-wielding, dark-channelling Fume Knight.
Note: There are four idols outside of the boss area--you do NOT want these around for the fight, because they actually heal the Fume Knight if he gets close. Use Smelter Wedges to get rid of them.
This guy wouldn't be too bad if not for his massive greatsword. When attacks in a combo with his smaller sword simply block or retreat to dodge the blows--the same goes for a thrust.
He usually follows this combo with a swipe from his greatsword. Roll under this to avoid damage--it hits hard. If you wind up on his left side, he may also swipe the greatsword at his side. Roll under to dodge this too. If he brings it over his head, roll to the side to avoid the vertical slash.
About halfway through the fight, the Fume Knight will imbue his sword with dark magic. If you can't dodge the shadowy strikes, you'll have to block and take a bit of elemental damage. When he swipes the greatsword horizontally--slower than he did before--roll under as usual. He will also plunge the blade into the ground on occasion. Back away, as he'll send out multiple dark orbs. With enough distance, you can easily sidestep them.
The Fume Knight fight requires careful dodging and patience. Bring phantoms if you need to, but prepare for a very difficult battle.
For more help be sure to check out our complete Dark Souls 2 Boss guide.