From Halopedia, the Halo wiki
This is a list of gameplay tips on Flood.
Halo: Combat Evolved
It is best to use the MA5B Assault Rifle, M90 shotgun, or M6D Pistol against the Flood. The pistol is capable of killing a combat form with a single shot to the chest, but is generally not recommended for mass engagement, as its medium rate of fire and small magazine size can lead the player into a position where they are surrounded by Combat Forms, therefore, it is best to only apply it in long-range combat. Furthermore, Combat Forms have a tendency to revive after being shot once with a pistol. The Plasma Rifle is also a good weapon of choice to use (if the assault rifle/shotgun are unavailable) as its high rate of fire can quickly destroy infection forms while still being potent enough to destroy the combat forms.
The Sniper Rifle is ineffective against the combat forms and will not kill them. The Shotgun, on the other hand, is the best choice against Flood Forms, as it usually finishes them off in one or two shots, especially Combat or Carrier Forms. On lower difficulties, Shotgun ammo is almost always available.
The Assault Rifle, an all-round effective weapon with large spread range, is superb against masses of combat forms and infection forms. Never bother to melee Combat Forms or Carrier Forms, as it is practically useless and very time consuming. For infection forms however, a single melee can take down four to five of them
All in all, the most practical option is to have a shotgun for close-up/quick work, and a Pistol or Assault Rifle for longer range. The pistol is most useful for the level Two Betrayals, while the Assault Rifle/Plasma Rifle is preferred on The Library due to the levels cramped nature.
It is possible to shoot the arms off of Combat Forms (alive or dead). Dismembered Combat Forms pose no threat. This works well on the level Keyes.
Splattering The Flood with vehicles such as the Warthog and Ghost is a good strategy, but it will receive some knockback if a Carrier Form explodes close to it.
Dual wielding SMGs and other dual-wielding weapons has proven to be very effective, as well as the Sentinel Beam, M90 shotgun and Energy Sword. Plasma Pistols, however, are the least useful. Dual-wielding Plasma Rifles, and aiming at the arms of the Flood is more than enough to rip them apart easily. The arms will come off quickly and therefore leave the combat form, quite literally, unarmed. They will waddle about and explode after a few seconds. There is also a way of killing combat forms in one shot. On the chest, you can see the infection form's tentacles waving about. This is where the infection form has burrowed. Shoot the infection form, and unless the combat form is utilizing shields, the combat form will die.
The SMG is also an excellent weapon against the Flood, as long as ammo is in large supply. Also effective is the BR55 Battle Rifle, as aiming for the Infection Form in the chest of a Combat Form will kill the Infection Form controlling it in a single burst. It is often a good idea to use the Brute Shot's melee, as it will have the same effect as an Energy sword on Combat Forms; it will destroy the body and prevent Infection Forms from reanimating it. If you don't have access to an Energy Sword or a brute shot, you can pick up a gun you don't need and use it to shoot the dead flood's arms off. Now it doesn't matter if it is resurected because it can not harm you. For one of the best combos, use Dual SMGs for Combat and Infection Forms, and keep an M90 Shotgun in reserve. Another highly effective combo is to carry a Battle Rifle for engaging the Flood at long ranges and carrying an M90 Shotgun for when you don't have enough time to be precise with your aim. A good strategy on The Oracle is to dual wield a pair of Needlers, you can usually do this without running out of ammo. Also, don't let the infection forms touch you if you have no shields; otherwise, they will kill you instantly.
Splattering the Flood becomes more difficult since the damage mechanics for splattering are changed considerably. The Flood can immediately hijack the vehicle if they survive a ramming attack from a driver.
The Incendiary Grenade instantly destroys all pure Flood forms and is the one true weakness of all pure Flood forms, as well as its Support Weapon counterpart, the Flamethrower. Also, all forms can be instantly destroyed with a melee, with the exception of the Pure Forms, which cannot be revived anyway when killed. There is also a way of killing Combat Forms in one shot. On the chest, you can see the Infection Form's tentacles waving about. This is where the infection form has burrowed. Shoot the infection form, and unless the combat form is utilizing shields, the combat form will die. However, the corpse will be susceptible to infection by another Infection Form unless the body is in fragments. Infection Forms can then revive whole Combat Forms if their body still resides on the floor. You can also head shot some combat forms with the Battle Rifle or a Magnum. Dismembering bodies is recommended to prevent Infection Forms from reanimated dead flood Combat Forms.
Plasma weapons are also very good to have when fighting the Flood, since the plasma melts the bodies. The Sentinel Beam is also very accurate when shooting the Flood because it also melts their bodies which kills them faster.
Splattering has become more difficult due to another change mechanic. Infection Forms now can give some knockback to the vehicle if splattered, depending on the vehicle that the player is riding.
Their tactics on Halo Wars are very similar to those they perform on the Halo Trilogy; they can obviously infect Humans, Elites, Brutes and now even Jackals and Grunts with their Infection Forms, and usually attempt direct attacks with large groups. They possess large tentacle-like Flood forms and large pods that can pin in and attack vehicles, in the case of the Elephants they capture, which is an in-game objective to rescue in the first level where they are encountered in-game. The Flood establish bases by capturing and spreading an infestation on enemy bases.
The Flood have been confirmed by Ensemble to have been made overpowering to both UNSC and Covenant forces to keep their appearance terrifying. Early in development, Ensemble considered making the Flood a playable race, but this idea never made it past the concept stage. According to lead designer Graeme Devine, the main issue was that in order to balance the Flood with the UNSC and Covenant, the Flood would have to have been similar to StarCraft's Zerg, which did not mesh with the Flood's role in the games, to be "the single scariest thing in the galaxy."
In combat, the Flood are similar to the Covenant, though with far more combat "buildings" than them. Countering them is also different from the Covenant; infantry units are discouraged (as they are easily infected and turned against your remaining units), and land vehicles are also a less than viable idea. On the other hand, air vehicles (especially the Vulture) are overpowering; few Flood ground units have anti-air capability, and those that do are, ironically enough, easily overwhelmed with superior numbers and barrages of fire, or flattened with Carpet and Cryo Bombs.
Halo Wars 2
Their tactics used in Halo Wars 2 are retained from Halo Wars; however, they can no longer infect Grunts. A unit will only be infected when it is killed(in the case of Grunt squads, the Jiralhanae or Sangheili squad leader will be infected) and a squad cannot be infected unless it is killed. The Flood now can infect vehicles that are still operational using the Flood Infestor, such as the Chopper, Warthog and Wraith. The process is similar to hijacking, except it won't spawn any Flood Forms when destroyed.
The Flood have become more lethal, as swarms of infection form can easily overrun a base with less effort; however, the tradeoff is that splash damage can kill them very effectively. A squad of Hunters or a single Kodiak can defeat hordes of Pod Infectors single-handedly. Turret defenses are no longer as effective at dealing with swarms of infection forms, even with maximum upgrades, with exception of the Mega Turret and Siege Turret. The number of Pod Infectors in a unit has increased considerably, being 3 to 4 times larger than those Halo Wars.
New Flood forms are introduced, such as the Flood Spawner, a mobile form that can produce Flood counter units depending on what type is attacking it. During gameplay, the Flood can also use Flood burst pods to emerge from the ground, which can be used as a distraction and as a surprise attack.
All vehicles are generally "downgraded" when infected, as they cannot use vehicle abilities. Additionally, the Flood also cannot infect any non-combatant vehicles such as EV-44 Nightingale.