From Halopedia, the Halo wiki
Looking for the gameplay tips for the Flood in general?
The Pod infector is one of the easiest creatures to eliminate in the Halo series, requiring only one shot from any weapon. However, as easy as it may seem, a swarm of infection forms can also be a formidable threat to the unwary individual.
In order to overwhelm hosts, infection forms attack in large swarms, leaping at any nearby targets and attacking with their claws and tentacles. If the victim's shields are active, as explained earlier, the infection form will burst. However, the sheer force of numbers can sometimes deplete a victim's shields, allowing these Flood forms to take control of the host's body.
While individual infection forms are weak, in large numbers they can exert a surprisingly powerful force, being able to shatter doors made of Forerunner alloys.
A single round from any weapon will burst an infection form. For this reason, automatic weapons are recommended. Dual-wielding Plasma Rifles, Brute Plasma Rifles, Spikers or using a UNSC ballistic weapon all work well. The Battle Rifle with its bursts fire can be quite effective and the Shotgun can work well too when shooting at large groups closing in. Try to avoid using weapons such as M6 Handguns, as their rate of fire is very low. Also, try to use Grenades, but be careful as sometimes Infection Forms can be stuck with Plasma Grenades and may harm you when it gets close. In Halo: Combat Evolved, automatic weapons like the Assault Rifle and Plasma Rifle are good on them as well as the Plasma Pistol, and the Needler can turn out to be a very good alternative as the needles have homing ability. Be careful though, do not stick too many Needles on one at a time while they are close as if you do it will explode and effect you most likely depleting your shields. The best way of using it is to fire a single shot at each Infection Form, which is enough to kill them. SMGs are incredibly effective for taking on swarms of infection forms with its large magazine size and impressive rate of fire. The Sentinel Beam is also extremely effective and can swiftly obliterate entire swarms of infection forms at long ranged distances.
Infection forms are spawned from bulging, sac-like pods in Flood hives, called Flood blisters. It may be tempting to destroy these sacs, but it is actually more advisable not to, since destroying them will not kill the infection forms inside.
Halo: Combat Evolved
Halo: Spartan Assault
Halo Wars 2