The Hell Guards is the second Boss in the game. You fight them at the end of Mission 11: Necropolis. The fight consists of two phases. In the first phase, you fight only one of the Hell Guards. He has a shield and you can only damage him when the shield is down. Use a weapon that deals a lot of damage with each hit. The Gauss Cannon with Precision Bolt mod is recommended. Try to get headshots while using it. When he jumps at you, there will be some fire on the ground -- so make sure you jump when he does to evade the attack. Basically, all attacks the Hell Guards do can be evaded by double jumping (in both phases of the fight). In the second phase, there are two Hell Guards, but they do not use shields -- so you can kill them very quickly. When the second phase starts, use the BFG on the two enemies. This will take away a large portion of their health. The Heavy Assault Rifle with Micro Missiles mod deals a lot of damage. It also causes the enemies to drop health and ammo. Simply run around them in big circles and shoot rockets at them to quickly kill them.