There aren't too many differences between this new Dragonrider boss fight and the first one you faced - other than the minor fact that this time around there's two of the blighters to deal with. Just be aware that one of your opponents will use a bow against you, and hides in the corner of the room for most of the fight, while the remaining boss is as good as identical to the original Dragonrider.
How to kill both Dragonriders
We recommend taking down the ranged attacker first, although you'll need to lure the other boss to the far side of the room in order to do so. Whenever he fails to land an attack against you, race over as fast as you can to the ranged enemy, and throw as many whacks at it as you can manage. If you stay very close to the bow-wielding warrior, it won't be able to fire its arrows at you.
Always keep a close eye on your other opponent, and move out of its attacking range whenever it approaches. Keep this strategy up until the ranged opponent is dead. Now you can turn your attention to the melee boss.
Just strafe all the way around the boss to avoid the worst of its damage, or you can dodge out of the way to avoid the incoming attacks. When it's finished launching its assault, you should be able to get two or three hits in, depending on the speed of your weapon. Keep all this up and you should have no trouble finishing off the last boss.
How to find King's Passage
When the pair of Dragonriders are dead, go through the door ahead of you and light the bonfire. Now carry on down this hall and take a right when you reach the small round room. Go up the ladder here, then turn around and walk through the gate.
Watch out for the statues in this next area, as they'll come to life and attack you. Just make your way through the room very slowly, and you should never have to deal with too many statues at the same time.
"Watch out for the statues in this next area, as they'll come to life and attack you."
Once you've killed all of the statues, go through the door in front of you and interact with the coffers in the next room to loot a Caitha's Chime, a Soul Greatsword, and an Old Knight Hammer. Carry on through the next door along, then make your way up the big set of stairs.
Turn right when you reach the top of the stairs, then carry on down the hallway and kill the pair of archers you come to. Keep a close eye out for any incoming danger from the archers who are over on the other side of the area while you're do this.
When you reach the end of this hall you'll come to a coffer. Loot it to add a Firestorm to your inventory, then make your way over to the other side of this area and kill the pair of archers.
The chest nearby is booby-trapped, and you'll be staring down the barrel of a volley of heavy bolts when you open it. Dodge or block these shots, then rummage around in the chest to loot 10 Fire Greatarrows.
Go back downstairs now and kill the trio of enemies lurking around at the other end of the room. Now go through the doorway, kill the enemy in the next area, then go downstairs again and walk through the door. You'll be able to loot 10 Flame Butterflies and five Repair Powders from the next room, as well as an Estus Flask Shard from the nearby chest.
Interact with the lever to open up the gate that's over in the corner, then go through the door and walk down the stairs to get back to the bonfire. Once you're at the bonfire, go through the door and keep walking along until you come to the circular room. Walk onto the plate that's in the middle of the room to get a lift up to the next floor.
If you go through the doors in front of you you'll come across a prisoner. You can't do anything with him until you've looted the Key to the Embedded, so just keep this location in mind for now, and come back later when you have the key - you'll loot it from the Demon of Song. When you've got that key, you'll be able to loot a Ring of the Dead.
For now, simply open up all of the coffers you can see to get a Strong Magic Shield, a Fire Seed, a Soul Vessel, and the Key to King's Passage. Make your way back to the lift that brought you up here, activate the lever, and take the trip back down. Now you can use that key to open up the door in front of you. Once opened, walk along the hallway and into King's Passage.