1. Increase your strength, the higher the better.
2. Find out where she starts feeling pain and stick to right outside that.
3. Often see if her pain threshold has increased and repeat step 2.
4. Try to keep her pain below 50%
5. Last 90% just push through the pain to get that last 10%
Not a fool-proof plan at first but with the increases of strength you should eventually pull it off.