It's like you can't fight your opponent because even if you kill him, you'll probably be low and just get ulted along with your whole team as the words triple kill show up on the screen.
That's why you've got to pay attention to where is he as he dies. Once he's dead disengage immediately.
It's not dodge able so idk what to do.
But there are things that can negate it. Soraka makes it kinda useless, Edge of Night will proc before his Requiem, Banshee's Veil will block it, Locket of the Iron Solari on a tank should shield most if not all of it etc.
Not to mention Requiem's cooldown is incredibly long (200/180/160 seconds at 6, 11 and 16). With max CDR (45%) that's 110/99/88 seconds. Once he's used it, you don't have to worry about it for a while.