Try to catch Vivian off-guard. Remember that she has no way to escape, so if you catch her before she can lock onto you, you have the head start in damage - and for Moji, that usually means the kill.

Moji has a hard time aiming around Vivian's shield, so to deal with her, flank. Be weary of sensors and If Vivian placed one in the flank route, take it out, wait a bit and strike later once Vivian assumes you've left and busies herself.

When Vivian shows up with her shield, Moji is useles, and you don't have any chance against her. Try to run from her and wait until her shield is out. But if you buy wrecker you can easily break her shield and beat her timing your magic barrier.