he has a 5 min engrage timer. you need two tanks. its been done with 1 very geared healer. from wowwiki
On normal difficulty, with Archavon's health at 2,300,900 HP, the raid as a whole needs to output more than 7,670 DPS. As a good rule-of-thumb, with two tanks and two healers, the tanks should never go below 550 DPS and the remaining six members should stay above 1,100 DPS.