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.
As a frontline, Viktor's Barrage will be blocked by your shield if the center of the blast is outside your shield. The missiles are just a visual effect, and the explosions actually spawn at ground level.
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.