For automated missions, you want the success chance to be as good as possible with as few survivors as you can (once you reach the "High", or even better, the "Very high" chance, you can send them).
I don't know what kind of weapons and gear you have, but I found that sending two survivors, one with a gun and another with a melee, gives you a better chance than sending either two guns or two melees. Try to equip weapons with high damage per second, since they give you a better success chance, and make several combinations to see which ones are the most efficient for your survivors, weapons and gear.
indeed, medics cannot heal in auto missions. Engineers are good melee fighters, but not as good as a fighter or recon; their utility comes later in the game, when you want to craft certain items, upgrade some buildings and disarm traps.
Edited by Lana Woo, 08 November 2016 - 02:55 PM.