From Halopedia, the Halo wiki
Skirmishers play an intermediate role between the cannon fodder Grunts and the powerful Elites and Brutes. They most often play a harassing role in combat, supporting heavier units with fast attacks in multiple directions. Skirmishers are dangerous both in long range and short range combat. They tend to be very accurate shots. Their most distinguishing feature is their speed; they can outrun any other Covenant species, and even Spartans. They also make great use of their jumping abilities, which are comparable to those of the Flood. They can leap up into buildings to escape enemy fire or to assault their occupants, and rush to capture strategic locations on the battlefield. Their armor can withstand some small-arms fire, making head shots the most effective means of killing them.
The Murmillo rank of Skirmishers uses elbow-mounted point-defense gauntlets for additional protection, and the Commando and Champion ranks will use hologram equipment to misdirect their foes. Their tactics utilized include constantly strafing targets, and often alternating use of their two point-defense gauntlets to let the other recharge. These gauntlets, while not nearly as large as those that Jackals wield, are used very effectively by Skirmishers to protect themselves.
Skirmishers do have their weaknesses. They are not issued with grenades and cannot board, making them relatively helpless against vehicles. They can only rely on their agility to avoid being splattered. A Skirmisher is also no match for a Spartan's strength in hand-to-hand combat, and in game they have no melee ability, making them just as vulnerable to being beaten down as their lesser Jackal cousins.
Changes from Halo: Reach to Halo: Infinite