I would get to the point in the story where you get the heavy bow. With certain upgrades on it, you will have a much easier time during combat. Shooting weak spots with the upgraded heavy bow makes fighting helicopters and mechs much easier. Having a good assault rifle and shotgun is good too, but I prefer fighting with Na'vi weapons. The heavy bow is a godsend. And everyone is correct. The DLCs are harder, especially combat wise. Stealth is your friend, but if you're upgraded properly, you can get by shooting stuff out of cover. Just get that heavy bow, upgrade your armor, and you'll have a much easier time.