He has a huge 360 degree knockback which seemed random. If your fighting him "on" the island your likely to loose a large portion of your raidforce due to knockback, as many will get knocked into the sea.

Once your locked in combat you can no longer climb back up the wall, making these players rather useless for the remaining duration of the fight.

Suggested that the raidforce be placed in the water about 20 feet from the island. The MT (or player you designate to pull) will climb up, pull Siyamak and jump into the water.

The tank will then move so his back is against the island and the raidforce will engage Siyamak from the rear or his flanks.

When down to 30% health Siyamak will attempt to 1-shot your tank, so get those heals and wards going big time. Siyamak also memwipes at this point.

Once the MT re-establishes aggro, the rest of the fight is easy.