Viktor is not CC immune during Barrage! If you so happen to be close to an enemy Viktor using his ultimate, Runic Blast will prevent him from attacking for a few seconds, allowing you and your teammates to save themselves and/or kill the Viktor.
Terminus siphon annoying you? You can ult him, which gets rid of his siphon and have your team burst him from there. Your gourd is on the ground, which damages him even if he has his siphon up.