Viktor has no burst damage to finish off wounded enemies nor burst mobility to catch up to fleeing targets. he can output massive damage, but will struggle to get kills the moment a target hides behind a wall.
Viktor's ultimate is reliant on him having direct vision of you so moving out of sight behind a corner or using opaque deployables(such as Inara's wall) will prevent him from selecting your location.
His ultimate will completely ignore shields (Including Barik's ultimate) and instead will instead do direct health damage. Find a blindspot instead of trying to tank the damage as a frontline champion.