From Halopedia, the Halo wiki
In battle, Jackals share many similarities with Grunts, but are far more intelligent. Like the Unggoy, they are a weak subordinate race and are somewhat cowardly. Despite their efforts to assure the Covenant of their superiority over the Unggoy, they often cower in battle and retreat with their shields up in the air.
Jackals are mainly seen wielding Plasma pistols and Needlers, but they may carry Type-50 particle beam rifles, Plasma Rifles or Carbines. Sniper Jackals are equipped with a Beam Rifle or a Carbine for maximum damage and range as seen in Halo 2 and Halo 3 respectively. Jackal marksmen are also equipped with a plasma pistol which they will switch to if in close quarters with an opponent, however most of the time they turn around and retreat.
The Jackal's Point Defense Gauntlet is almost completely impervious to all human firearms, and is strong against plasma based weaponry, but if a plasma weapon is fired at it for an extended period of time it will easily deplete the shield. The various ways of combating Jackals basically works like this: get an accurate weapon; such as a Carbine or Battle Rifle, and then fire at the Jackals exposed side, the Jackal will recoil in pain and move the shield away to leave the body bare, leaving the player free to perform a head shot. Another good way is to overcharge a Plasma Pistol and fire it at their shielding, deactivating it, giving you the time to fire at them or melee them. In the later games, meleeing the Jackal will end up deactivating the shield, and sending the alien fleeing. In some occasions, the Jackal may stand its ground and attempt to kill you.
The Jackals were revealed to have a new role in Halo 2 as the Covenant primary marksmen. The use of marksmen Jackals changes the game drastically, because up until now, all Covenant fighters were close range fighters, none of whom carried scoped weapons, and now the Covenant are utilizing tactics seen in Human warfare, in this case, more ranged combat.
Killing Jackals at range becomes much easier in Halo 3: ODST, due to two new available weapons: a more powerful Covenant Carbine, and the new M6C/SOCOM (the larger magazine, coupled with a scope almost as powerful as the sniper rifle's, makes a devastating combination). In both cases, the tactic is the same (the M6C is slightly better, as it is somewhat more accurate)- shoot the Jackal on its exposed right arm (the one with the plasma pistol, needler, or plasma rifle). The Jackal will quickly move its arm to a little bit above its head- but only temporarily. While the Jackal is exposed, get an easy headshot. Sniper rifles and particle beam rifles are one-hit kills if the player hits the exposed arm.
At close range, the tactics change somewhat, depending on the weapon the player is using. Always keep in mind that it is usually best to snipe down a few Jackals while they are charging at you, before the Jackals get so close as to make sniper weapons useless. A frag grenade under the energy shield can kill a group of Jackals with the overlapping energy shields. Brute shots are also effective- even on heroic or legendary, one grenade from the Brute shot can kill a Jackal. The noob combo (plasma pistol/rifle + assault rifle) is also effective (shoot down the shield with the plasma rifle, then kill it with the assault rifle. Shotguns are also effective at close range, and like the sniper rifle, a shot in the arm will kill the Jackal. If approaching a solitary Jackal from behind, keep two things in mind: 1. Don't waste your ammo for a close range kill- melee.
Halo Wars Stat
In Halo Wars, Jackals play a powerful anti-infantry role, having been given the capability to inflict a number of casualties upon regular enemy infantry that are disproportionate to their own numbers, much like the UNSC Hellbringers-though having the advantage of longer range weapons. They are deployed in groups of two, and their default armament is the medium range Covenant Carbine. However, being geared towards a single role, the effectiveness of Jackals against armor and aircraft is highly limited, and will easily fall victim to vehicles. However, Infantry do, generally, defeat aircraft and the Jackals, in groups, will easily destroy enemy aircraft. Like all units in Halo Wars (Excluding the Vulture and Scarab), Jackals have multiple upgrades as listed below in the precise order researched:
Changes from Halo: Combat Evolved to Halo 2
Changes from Halo 2 to Halo 3
Changes from Halo 3 to Halo Wars
Changes from Halo 3 to Halo 3: ODST
Changes from Halo 3/Halo 3: ODST to Halo: Reach
Changes from Halo: Reach to Halo 4
Changes from Halo 4 to Halo 5: Guardians