Pop a bubble with weapons of light across the room from where Valus comes out. All 3 guardians use bubble and unload their rockets on Valus. Immediately pop an ammo synth and unload again. Run up top to the right and hit Valus with snipers or fusion rifles. He won't have much health left at this point, and will be dead in a couple of minutes tops. Just keep moving so you don't take too much damage.