In a 1v1 situation, silence him with running blast and deal damage until he uses emitter, then channel recharge towards him until he runs out of shields then finish him. Nimble would be perfect to keep pressure on him if he tries to run away.
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.