From Halopedia, the Halo wiki
Drones prefer to stay at a distance and use long-range tactics; however, they are skilled close-range combatants as well if the need arises. Since they appear in swarms, they can often confuse or disorientate the enemy, which gives them an advantage over fewer, slower-moving enemies. Drones also have the distinction of being able to board vehicles. They do not seem capable of driving them, but will claw mercilessly at the driver until he is killed or gets out of the vehicle. However, their melee is weak, you can simply wait it out until someone shoots it down or simply crash into a structure, causing it to lose its grip on you. Their capability for limited flight allows them to easily weaken an enemy and retreat as necessary when in danger.
They wield lightweight weapons such as the Needler and Plasma Pistol, though they never overcharge their Plasma Pistol to break a shield, as with Jackals. They also enter battle in a swarm (the number differs depending on the difficulty); furthermore, they are susceptible to headshots, though sniper weapons will always kill them with a single shot regardless of where it hits them and the difficulty level. This is most likely due to a natural physical weakness, as the sniper rifle's shot would likely travel straight through the exoskeleton and cause the Drone to go into a state of shock. Drones never dual wield or have heavy weapons because they can only become airborne while lifting a small amount of weight. When fighting, they have to take frequent rests while in the air, probably due to the gravitational pull from Earth and High Charity. They are said to have replaced the Elite Rangers after the Covenant Civil War began, probably due to their inherent maneuverability and incredible aim. Even though their weapons aren't very effective, be wary of their precise aim because too many hits, even from a weak weapon, can be fatal. It has been noticed that the Spiker and Assault rifle are the best weapons against Drones, and when dual-wielding anything, you can literally shred your way through a swarm.
On Easy and Normal difficulty, the Drones are easy to kill and present no real threat; however, on Legendary, they can be an extremely deadly enemy to face. Their ability to fly means that if you have cover, they can still fly over and around you, and due to their pack mentality they can outnumber and kill you very quickly. Individually, however, they are still rather weak in resistance. One tactic is to lead the pack into a narrow corridor and throw any kind of grenade at them. Most of the time this tactic will kill a majority of the swarm.
In Halo 3, Drones have the ability to lift Marines from the ground, similar to the Brutes. The only time this has been witnessed is during scripted events, and if you kill the Drone in question it will let the Marine go. They have also kept the ability to board vehicles and melee the driver.
There is a moment in the Halo 3 level Tsavo Highway where approximately twenty Drones swarm out and attack you and your passengers in whatever vehicle you currently have. On Easy, Normal, and sometimes Heroic, the Marines will shred through the Drones with minimal fuss, but on Legendary it is a battle to be reckoned with. In addition, on the level Crow's Nest after the bomb is activated, a huge swarm of fifteen or so clumped-together Drones will fly past you. If you throw a plasma grenade into their midst it will usually stick one and kill most, if not all of them. This will incite the wrath of any nearby Grunts and the remaining Drones.
Drones are weak, even more so than Major Grunts but about as strong as Minor Grunts. However, in large numbers (as they travel in swarms), they can be more deadly than Brutes. First, find cover, and then size up the situation, think about the odds. If you recklessly charge in headfirst, you may take out some of the buggers, but your shields will be overwhelmed, and you will be cursing your fate as you see the Drones shoot your corpse. Then, toss your more powerful weapons (not too far, as the point is ammo conservation) for an SMG or plasma rifle. In addition, you may wish to get a Battle Rifle. First, through a hole or crouching, snipe a few out the air. Then, duck and wait for them to forget about you. Repeat this as many times as necessary. Then, switch to your SMG for cleanup and pick off the few lone survivors (shouldn't be too arduous). The main essential tactic, though, is to use fully-automatic weapons. Sniper Rifles will maybe only take down a few out of dozens, and plasma grenades might as well pass through them like thin air. Assault Rifles and SMGs are the best common weapons against them.
Drones were first seen in the game Halo 2, after a long period as combatants during the Battle of Earth, and were featured often in overwhelming swarms in open spaces. They had less distinguished, shinier, chitinous armour, and much smaller, luminescent eyes and a less insectoid and more alien-like appearance.
When in large spaces, the best weapon to dispatch a Drone swarm is to use the BR55 Battle Rifle or the Covenant Carbine due to the ability to use the scope. Use the Human turret if it's available, but if on higher difficulties, this is recommended only if one knows the origin of the swarm. When close to a Drone, the M90 Shotgun also works, but it would be a waste of ammo since the player can just melee the Drone itself.
The best tactics for dealing with a Drone swarm is to hide behind cover and take the Drones down one at a time. Also, if a Drone boards a vehicle it will claw at the driver until they are killed, a certain amount of time has passed, or they themselves are killed. An effective, but dangerous way to remove them from vehicles is to ram the vehicle into a nearby wall or large structure. This does, however, risk the destruction of the vehicle and possibly death.
On the level Crow's Nest, if the player is vigilant enough, he/she can take and save a Deployable Cover from the hangar. After the Drones grab a Marine (seen through the mesh), run to the left and place it in front of the .50 Cal Turret. Doing so will enable one to shoot through and kill while avoiding the incoming Plasma Pistol shots from the Drones. Since Deployable Covers are commonplace on Crow's Nest, all the player needs to do is to seal up the pipes and keeping them from coming out. It should also be noted that drones are extremely vulnerable to melee attacks, and on most difficulties one hit is enough to kill a drone. The trick is to either corner them, or wait until they have just landed. Simply run up to them and deliver the fatal blow.
One more simple and effective tactic to deal with Drones is when they are in a swarm. All that is required is a plasma grenade to stick one of them in the center, or an explosive (such as the Rocket Launcher or the Fuel Rod Gun) and most, if not all of the swarm, should be wiped out. The projectiles Brute Choppers fire can easily kill drones in one or two hits. This tactic can be used while sitting outside the first blue energy field on the level Tsavo Highway.
Halo 3: ODST
In Halo 3: ODST, there have been many changes to the Drones. There are now multiple color schemes (and, presumably, ranks related to them) for the Drones (including gold-shelled ones), and some have been seen to equip weak energy shields.
Drones have also received a running animation, featured prominently in the mission Data Hive. There they have been seen feasting on the corpse of Marines, and while they are sleeping, they do so hanging upside down on the ceiling, similar to bats. When they hang upside down, they are most vulnerable. Try to take out the highest ranking one you can see first. Alternativly, sneak through without firing a single shot; sleeping Drones will not wake up unless one is fired upon.
Weapon wise, the Silenced SMG is surprisingly effective. The scope allows for easier acquisition of the mobile Drones, and its incredible rate of fire means shooting them down is much easier. The Carbine (and, to a lesser extent, the Silenced Magnum) is also effective, as a single headshot will instantly kill any Drone. Additonally, for the first time in any Halo game, the Flamethrower is usable against non-Flood enemies, and proves its worth against Drones. A single burst of flame is enough to kill even the highest ranking target. Simply wait till they land and burn them up.