Haigarth has to be accessed from the Cave of Time, the one at Tanaris, there is a portal in Dalaran that will take you directly to the entrance of the Cave of Time, then talk to a bronze dragon who will take you deeper into the Cave of Time.

Bosses have to be killed, you can find 2 or 3 friends to go together or shouting at people is fine, Haisan has T6 and Argus, the group should be relatively simple.