If you're a ranged character or sorcery/miracle user, you can land a free hit on any of the dragonriders as soon as you enter. The dragonrider on the top of the ledge shoots arrows but has a much lower HP. You can fight defensively and wait for him to drop to the lower ground so you can prioritize attacking him.
Their attack patterns are basically the same; if you have a good shield that can block their attacks without taking damage or losing that much stamina, its quite easy to circle around them and counterattack. They're most vulnerable when they're using their shield bash and when they're swaying their weapons. Concentrate on taking out one target since the battle will become much more easier once one of them is down.