In Large Terrain, when going to save someone that is in the middle of nowhere with snipers everywhere, pull out your "syringe". Crouch, then run towards him. When you are near the person, run over him holding Fire. Keep circling him and you should have saved that person.

For Overload, get a spawn point and wait for some snipers to spawn. When they die, get on the ground and crawl to the soldier. By doing this you probably will not get killed. For Shadow Of Death, try to go to a spawn point with high hills. By doing this you cannot be seen. When a sniper dies, use the method described earlier and you should have some easy saves.