Difficulty: Easy
While you'll probably get this in normal play, you can speed it up by taking away all Sheva's ammo and leaving her to fight the enemies herself, while you dodge and run.
When she's seriously injured and dying you'll have to get close and follow the on screen prompt to save her. Repeat once more and heal her or she'll die the third time and you won't get the chance to save her.
Note: This could be boosted more effectively on bosses and higher difficulties.