From Halopedia, the Halo wiki
Unggoy Heavies are usually seen in conjunction with other Covenant infantry, operating as heavy support units and are usually operating heavy weapon systems such as T-42 or T-52 plasma turrets, Shade turret and Type-33 fuel rod guns with deadly effect.
They will usually not flee from their position unless the player is too close or out of range of their weapon, in which case they will pull out a plasma pistol and attack like a normal Unggoy. They are easily dispatched using most tactics. Their armor seems to be slightly tougher than the average Unggoy, most likely to protect them from explosives and the possible backfiring of their powerful weapons. Unlike most Unggoy, it is strangely rare for Heavies to throw Type-1 plasma grenades.
Note that the cannon is free for a player to use after taking out its operator. In Halo 2 an Unggoy Heavy must first deploy its plasma cannon before it can be used. If the Heavy carrying the plasma cannon is killed before it can deploy it then it cannot be used. This is not the case in games after Halo 2, since most of the plasma cannons encountered are already deployed, with the exception of a few. They can still be used even if they are detached from their tripod mount.
In Halo 3 and Halo: Reach, Unggoy Heavies are seen utilizing many different heavy weapons and vehicles. They are commonly seen driving Type-32 Ghosts and seem to be skillful drivers, especially on higher difficulties. Due to their low target profile, they are also much harder to kill in a Ghost from head on. Unggoy will attempt to ram enemies and will keep their distance from advancing foes. Unggoy Heavies may also work in teams on higher difficulties, in which case one rams a foe and another shoots the foe out of what they are driving. Usually, if they are not piloting a Ghost, they are manning plasma turrets or carrying a fuel rod gun. Like Special Operations and Unggoy Ultras, they are some of the most dangerous and tactical Unggoy. When not manning a vehicle or turret or carrying a fuel rod gun, Unggoy Heavies will usually use Type-25 plasma pistols, but on higher difficulties they may use Type-33 Needlers.
In Halo 4 and Halo 5: Guardians Unggoy Heavies will normally not use vehicles, with Sangheili manning even the Wraiths' plasma cannons. They will however, like their Halo 2 counterparts, enter empty vehicles or gun positions if given the chance. Like their Covenant counterparts they use Type-54 plasma pistols, Type-56 Needlers, and Type-58 fuel rod cannons. In Halo 5: Guardians, Unggoy Heavies will be somewhat more intelligent than the standard infantry unit. They will hold their ground for longer periods of time before retreating, and their armor is more resilient to body shots. They are also more resistant to melee attacks, as on Legendary difficulty, a spartan charge will not kill them although it will dispatch any Unggoy Storm. Regardless, one headshot from a precision weapon will still kill an Unggoy Heavy.
Halo Wars 2
Unggoy Heavy are exclusive to Yapyap and can be made at Raid Camps. Unupgraded Heavy Grunts do okay against infantry, vehicles, aircraft, and structures. Once upgraded with My Plus One Unggoy Heavy will perform well against aircraft. In Blitz Heavy Grunts cost 40 energy.