Lekgolo
From Halopedia, the Halo wiki
Hunters have very heavy armor that shields most of their body. Hunters are equipped with a large shield, equivalent to battleship grade titanium, and a fuel rod cannon. Hunters are generally called in for heavy defence and demolition. They work in bonded pairs and can be an extremly big problem, especially on higher difficulties. They are in fact not one organism, but many, many worms working symbiotically, held together by the armor.
The easiest way to kill a Hunter is to shoot the uncovered orange part of the body on it's neck, stomach, sides, and back. It is also possible to melee attack this spot for the same effect, though it it much harder and takes more attacks. It is left open, however, when it tries to melee attack you. It then staggers for a short while, leaving you a short window of time to turn around and shoot its back.
If you stand far away from the Hunter it will use its fuel rod cannon in an attempt to kill you, however the shot is slow, and easily dodged. If you are in melee range of the Hunter it may use its shield to bash you, resulting in heavy damage, if not death. If you stand behind the Hunter it will lift its shield into the air, then swing it back at you in an attempt to crush you. All of these moves are fairly easy to dodge but will make a devastating blow to your health if you get hit.
Grenades aren't very effective against hunters, and it will usually take a large amount of them to take one down.
In Halo 2, you first encounter Hunters on the level Outskirts. When you first encounter the Hunters, it is easy to use the machine gun on the second floor of the building, but this becomes harder on Heroic and Ledgendary difficulties, as the gun is very exposed and one shot from the hunters fuel rod gun and it will be destroyed. Another way to deal with the hunters is to take the sniper rifle from Sergeant Johnson, and shoot the exposed flesh at their back or when they raise their shield.