I have done it with a Hunter and a Warlock. Easier with the Hunter, because I could run through the first part, whilst being invisible. Used Ice breaker / LDR 5001(Void) to clear the 3 Lieutenants, but then hid behind the boxes as you enter the room with Valus. Moved and sniped him from there. If this is a "Cheese" then so be it. Took about 1.5 hours of painstaking shooting.