-It seems to me that you don't need to play every mission in order to keep the factions happy. I would ignore harvesters, and terror missions in the beginning. (harvesters are the ones flying with 1100 km/h).
-Also as the terrain, and objects are not destroyable, use as much cover as possible. Even a wooden fence will keep you completely safe from grenades directly on the other side (at least it worked for me).
-If you have the chance, only hire soldiers who have a weapon skill above 30, and make a small but good squad. You can build your tactics around the kind of squad you can hire, but snipers are a must.