When you encounter Nephtis at the power plant on Mars, grab a solar panel on one of the towers. Wait until she charges up her dash attack and guard with the solar panel. She will be stunned for a few seconds. Attack her until she starts moving again. Repeat this process until she is defeated.

When you fight Nephtis inside a giant force field, wait for her to charge up her dash attack. As soon as she attacks, dash out of the way and she will hit the force field and become stunned. Attack her until she breaks free of the force field. Repeat this until she is dead. Note: Be sure not to run into the force field while attacking her.

When Nephtis is inside the tower like area, use the Geyser to stun the Clods and grab them and then wait for Nephtis to attack. Guard with the Clod and Nephtis will be stunned, attack her until she starts moving again. If the Clod tries to break free use Burst to keep them from escaping.

When Nephtis's AI infects Ardjet, dodge any of her long range attacks, then when she is done move into melee. Wait for Ardjet to swing at you, then deflect her attack by slashing at the correct moment. Deflect the sword four times and she will become stunned. Grab and hold her while ADA deletes the AI. Repeat until she is defeated.