Starting with mission 3 you get a rating when each mission ends. The middle one is the Resilience rating and it's a measure of how few squads you loose on the map. to get a 5 star you simply need to keep every squad alive preferably with full troop strength, but that isn't required as long as the last guy is standing. Simply return to a beacon if you start to loose troops to refill and take a breath to recover health before pushing on wards to the next encounter and it will be yours.