I'm in a nasty situation on solo campaign where I have to face off against a pair of Mgalekgolo (hunter, mechanized infantry - the guys with the giant green ball cannons and shield) in a single-elevation room, and I can't figure out a strategy for killing them. I know I can assassinate them effectively if I can get behind them, but while there are a lot of obstacles in the room, I can't figure out how to get into that situation. Is there a way to kill them without using the knife? They laugh off grenades and I haven't been able to find a spot for a head shot with the DMR (I have access to a DMR, Machine Gun, Shotgun, and a few small caliber covenant weapons).
The answer appears to be 15 to 20 "face shots" with the DMR. I'm not sure if that's what just magically ends up being enough to get a head shot at distance, or whether they really start losing HP. The shots were basically directly to the face, they will bring up their shield immediately after (so don't just hammer on the trigger), and then drop it again and you can take the shot again. Just keep doing it until they fall over.
I generally have problems with hunters as well, but have had some success using a shotgun at medium or close distance. If you shoot them in the back with a shotgun, eventually the armor on their back will fall off, exposing more of their skin. They don't last long when their exposed area is hit with a shotgun.
Essentially all you can do if you don't have a power weapon or any grenades is keep unloading whatever weapons you have into them until they die. Try and concentrate fire on one of them so that you reduce the number of enemies firing at you asap.
When you damage one enough their backplate will fall off at which point try and concentrate fire on their backs to take them down quicker. Don't get too close to their backs (or at least don't hang around there) as they are prone to unleashing a devestating instant kill melee attack to behind themselves.
Armour abilities don't really do much good against hunters short or armour lock/drop shield to avoid one/two shots from them at the most. Jet Pack might get you out of the way but way too easy to get blasted out the sky. Sprint is useful for evading them, Active Camo next to useless and Hologram useful as distraction but will only take 1 shot from them to kill it.