Vivian’s only way to do damage to you is her gun. When the ultimate is up, she’s really dangerous.
Her sentinels have AoE, and their size is kinda big, too.
Try to focus her sentinels when her ultimate is up, so she isn’t so bursty.
don't try to repeatedly stun the enemy because the enemy can be stunned only every 3 second(unless you're using the wekono's wrath legendary) and that goes for every enemy. otherwise it's a good plan to use your stuns in 1v1