You don't. Ninjas that die are dead for good for the day's survival trial.

When you reset for the next day, they start fresh again.

thanks for the tip

If you know you are about to lose retreat and you wont lose any of your ninjas.

yeah, retreat when you see you are going to lose and change something in your talents/strategy. in early times, when the matches are hard, consider you can go through it by using more than 1 single lineup.
btw, two others tip: *
1) if you win a match at round 10 you still lose all your ninjas even if you pass the station. So retreat at round 9 if you see you are going to win with healings only.
*
2) healers and healing talents are your friends there.
This post was last edited by 988***@uf16 at 2017-4-16 21:14