Noxxion only does nature damage. He will primarily attack with slam attacks for about 500 nature damage. In addition to that, he has a number of other abilities:
Uppercut — Will knock you back a short distance and cause damage.
Toxic Volley — Around 300 nature damage initially. It continues to tick about 3 times after that for 14-28 nature damage.
Spawns — Periodically, he will disappear and spawn 5 Noxxion's Spawns. These smaller elementals attack for roughly 100 nature damage.
Notes
Despite the fact that he is a poison/nature elemental, he is not immune to nature attacks or poisons.
Strategy
Group strategy:
Because all damage is nature, plate armor does not help. You may want to consider using the character with the most hit points as your tank, which may be a feraldruid. When Noxxion's Spawns appear, you can opt to have a paladin with Consecration go in. It is difficult to hold on to aggro, and this is where groups will most often fail on this boss. Use AoE to handle the spawns, heal up if possible (bandages), and then concentrate on Noxxion when he re-appears again.