Not to be confused with the Dragonrider boss fight from Heide's Tower of Flame, the Dragonriders (plural) are a tad more difficult. Here, you'll fight two of the soldiers at once. When you enter the arena, there will be one directly ahead, and another on a ledge higher up; he'll be shooting arrows at you.
Kite the ground Dragonrider under the other's ledge, then beat him down quick. Both have surprisingly low health, but the one in black armour is substantially easier to kill. Once he hops down to join in the melee, focus him first. You can see how we beat this encounter in the video above.
For more help be sure to check out our complete Dark Souls 2 Boss guide.