These guards carry their torque bow weapon that shoots out bows that explode once they are attacted to the enemy (there are also a few guards that are armed with Lancers throughout the game). On the Casual difficulty setting the player can survive more than one shot from the arrows, but on Hardcore and above they are a one shot kill. The Therons make their first appearance at the end of Act III.
The Torque Bow arrows are difficult to dodge so staying in cover is a must when fighting Therons. The easiest way to kill them is to use a Torque Bow against them. This will kill them in one shot and you will get more arrows from their corpses. Sniping them also works fairly well, but it should be noted that headshots do not affect Therons. The chainsaw on the Lancer is a good bet if you are close enough to them as well.