Archavon the Stone Watcher is a quick raid encounter (like Onyxia or Magtheridon). This encounter requires two tanks.
This encounter with Archavon is a DPS race with only a few mechanics to watch out for:
Every so often he will start shooting rock spikes at a random raid member. This person cannot avoid the damage (around 7k). However, they can damage other people around that person. Just make sure you aren't unnecessarily damaging others or getting damaged by moving away.
He will also randomly jump/charge towards a random raid member and leave a white cloud on the floor which will do ~2k nature damage per tick if you are standing in it. After he spawns the cloud he will stand still for a couple seconds then run back towards the main tank. Stacking the entire raid within 10 yards of him will prevent him from using his Crushing Leap ability, but it is also likely to increase the splash damage from his Rock Shards.
Both tanks should watch their threat meter and taunt anytime they are not first or second on aggro. If the offtank maintains second place on aggro there is no need to time the taunt with the stomp ability, as the offtank will automatically get aggro when the main tank is picked up after the stomp. In raids where the off tank is significantly less geared than the main tank, one can have the off tank taunt right before the 45-second internal cooldown for Stomp expires, forcing the off tank to be picked up (minimizing the time he is tanking) and preserving the threat lead established by the main tank.
Archavon is on a hard enrage timer (5 minutes) that will boost his damage 8-fold and immediately wipe out the raid if he is not killed quickly enough. Upon initial release this fight was primarily a DPS race, although in current gear one should never encounter the enrage unless something has gone seriously wrong.
On 10-player mode, 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.
Effective tanking requires a swap in aggro following the Stomp ability where he picks up the primary target for several seconds and then throws them across the room. This can be handled in a number of ways, though taunts are required, because being picked up removes all aggro from that target. An alternative to the two tank method is to have a highly survivable dps grab the boss with a taunt and live through the several seconds until the tank taunts the boss back. A lesser strain on the healers can be achieved with taunting before the stomp is cast and thus being picked up instead of the tank, though the 0.6 second cast time on the ability makes this very challenging.
On 25-player mode, with Archavon's health at 9,970,700 HP, the raid as a whole needs to output more than 33,236 DPS. As a good rule-of-thumb, with two tanks and five healers, the tanks should stay above 850 DPS and the remaining eighteen members should stay above 1700 DPS.