Despite his name, Dragonrider isn't as intimidating as an armored knight actually riding a dragon might be, but he is sure to give you some trouble if you aren't using the proper battle tactics. Circle strafing is key here, and don't forget to keep that shield up.
To make the battle easier, be sure to clear out the giant stone knights in the area leading up to his arena, then pull the bronze levers to add more floorspace to his den. Or, if you're a ranged player, you can even cheese him into thrusting his halberd so hard that he falls off the edge. Watch as we break down the entire boss battle right here.
For more help be sure to check out our complete Dark Souls 2 Boss guide.