Keep your snipers alive. I have a sniper with a plasma rifle and it kills everything.
Like, i can be on the other side of the map, and still have an 85% chance to hit with a critical hit average of 15 damage. So i can basically one hit kill almost any enemy, and if the enemy isn't in cover i can take another shot. There was one instance where there were like 10 aliens within view, and i killed 6 of them and had to reload