Another tip you might add:
You c*so switch out a ninja from your survival team at any point if he hasn't been in any battle yet. (Is marked with a lock icon on the ninja icons at the "Select Ninja" menu)

Another tip:
If you have unlocked Trial Breaktrough there is a very easy way to heal up a team.
You can do the first trial, which is usually a weaker team, place in a healer, kill their most deadly ninjas and then stall to get your team healed up.