Spread out your team to avoid splash damage from Valus' missiles. Have them get on high ground to target Valus' head easier. Wait until Valus completes his entrance animation, which happens when he is done firing his gun wildly When Valus moves from one end of the room to the other, avoid attacking him as there will be a series of incoming missiles. Use the large boxes on the upper balconies as cover from his gun and missiles. You must also deal with the Phalanxes that spawn from the sides.