Focus fire, warp (best attack in the game against armoured opponents), reave, incendiary, incendiary grenades, armour piercing, incendiary and warp ammo, sniper rifles, hand cannons and assault rifles. Keep as much distance as you can. If you can keep him at a long range then it might be best to take out the collectors first and keep running and gunning until you grind him down. If he gets close and traps you then pull out all the stops, focus on him and make sure he dies asap. You might also want to spread out your squad if possible.