In addition to what others are saying, there's two key points to impale that make it somewhat trivial to beat:
1. The percentage chance of an insta-kill increases as your health drops in a 1:1 ratio. So at 50% HP there's a 50% chance for it to work. At 25% HP there's a 75% chance for it to work, and so on. If you're at full HP it's guaranteed to fail. Keep your HP high (DEF support characters, SHIELD STANCE skill, food items, etc.) and you won't have to worry about it.
2. You can block it. If impale does 0 damage to you then it won't insta-kill.
Hope that helps out other people struggling with Ophelia! :)