don't enter combat at all. An easy way to get around this mission is to complete the objective for not killing anyone without being detected and do the 3 easiest missions that way then just complete the mission and replay it to get the last objective to do all 5. You do have to get both optional objectives at the same time.
The first time I tried this mission the random events to free salves were set up in such a way that it was impossible to get away with out being spotted. I just finished the mission and came back later and it was a lot easier this time with the more favorable spawns.