Unggoy Heavy
From Halopedia, the Halo wiki
The Gunner Grunt's are a class of Grunt within the Covenant. Gunner Grunts are clad in green armor and operate Plasma Cannons and other fixed weapons used on the ground by the Covenant. These grunts are of high rank and sometimes operate in Special Operation teams. Those who operate in Special Operation teams usually carry fuel rod cannons
Gunner Grunts are a new addition seen only in Halo 2. They also seem to opperate in pairs.
Weapon Carries
Weaknesses
They are about as tough as Major Grunts. Melee them from behind, or use a Pistol or the Battle Rifle just as you would any other grunt.