is to expect losses.

Its an alien invasion, you're learning your enemy and how to react to them so while it may be frustrating you can expect some losses when their tactics change a bit.

I solved this by bringing a rookie with me that I didn't mind dying if it came to it. Send them closer up front to explore but have someone watching their back so they don't get wiped off the map. A leveled up sniper or two in the back with an ability that can fire multiple shots if someone comes into range and tries to attack your guys can be the best way to counter some of their cheap shots.

It gets to the point when you stick with it your stronger guys can handle anything the game throws at you. I think if you fire it up and just go with the randomness you'll be fine.