At the start of each round, you can use your first setback at her, sending her way back and having her walk across a big path without any movement ability. If she plays Mother's Grace, act quickly, before she goes into cc immune state.

If you are playing Stunning Visage, wait for her shield to come up then stun her, which completely shuts down her shield. Then, she is an easy target to burst down.

When he throws broadside, you can feed your power siphon.

If playing Stunning Visage, when you stun Viktor he has his head up, so try to headshot your shots when he is stunned and an extra 600 dmg after going through him are enough to kill him.