I've read a lot of other people's combat strategies and I will admit, those are for braver people. I tend to fight methodically, which often means slow and stealthy. With the practice, I've actually found playing on Heroic Mode now to be easier than the first time I played on Normal. I'm not the best at FPSs (a supreme understatement), so these strategies are really for my brothers and sisters that just don't have the knack for this kind of thing but still want to play.
Almost not worth wasting ammo on. No more than one grenade to scatter them, then hit and run with melee attacks. If they're accompanied, it's almost mandatory to take out their escorts first. However, you could also pop 1 to 3 pistol shots in them from a distance. Save sniper bullets for the Elites and Jackals, unless of course you find a couple lined up. I'll never forget the time I took out three Grunts and a Blue Elite with only one shot from the sniper rifle.