From Halopedia, the Halo wiki
Floodgate is essentially a reversal of The Storm. Players will have to backtrack from the Covenant Anti-Aircraft Battery to Lakebed A, fighting their way through a massive Flood infection. The level is the first Flood-only level in the game, introducing Combat Forms, Pod infectors, and Pure Forms.
Finding your way around Floodgate shouldn't be much of a problem—you're backtracking your footsteps all the way to Lakebed A in the previous level. This is the first mission in which you fight the Flood, making it a new experience when fighting the newly evolved Pure Forms. Keep in mind the useful weapons when in combat with them and any other Flood, especially on higher difficulties. This is a good starter mission for engaging the Flood. You can test the best technique of dispatching them without dying, because although there are some instructions about where to go and what to do, there are a billion other techniques that work just as well.
The recommended skulls on Legendary are Tough Luck and Catch. Tough Luck may cost you a few multi-kill bonuses, but grenades aren't a vital part of your arsenal during this mission. Catch is, as in all other Flood-only missions, a consequence-free multiplier—the Flood don't use grenades, so leaving it inactivated is a waste. All other skulls except Famine are okay on this mission, so try activating one of them too. You may not want to activate Fog if you aren't skilled at close combat with the Flood. Due to its short length, this level is arguably the easiest level to beat with Iron activated. Mythic is a consequence-free multiplier, as there are no Covenant enemies present on this level. Tilt is not recommended on this or any other Flood level.
New in Floodgate
It Followed Me Home (Mission Start)
If you're skilled with the Battle Rifle and are trying to get the Exterminator achievement, you may want to look to the right of the path that is about to be described. There, in the distance, if you are quick enough, you may see several Flood Combat Forms assault a small group of Marines. They will begin to retreat, though if you fire at them, they may turn to face you, and strafe instead of running away. For quick kills without using a lot of ammo, aim for their chests to kill the Pod infectors that reside inside of them. When you're done with that, proceed as you normally would...
Backtrack down the path you took to get to the Anti-Aircraft Battery. Make your way down the hill, and enter the Storage Area where you previously fought the Brute Chieftain. There will be a building on your right. Just after the checkpoint—right when a Marine says, "All squads, report!"—a Flood Combat Form will appear on the roof and jump over the street. If you kill this Flood during his jump (or right as he starts to jump), he will drop the Fog skull. Collect Frag Grenades, a Gravity Hammer, and a Shotgun en route to your first encounter with the Flood.
As you approach the Marines—some of which are gathered behind a pile of sandbags, standing next to weapons lockers—Combat Forms will drop down and start attacking your allies. The safest course of action at this point would be to hang back and wait for the Flood to approach you. Jumping in to save the Marines is a risky decision—seeing as a single Pod infector can make easy work of one, their chances of survival are close to nil, so rushing in an effort to save them will only leave you surrounded by enemies. Make sure you have your Gravity Hammer at the ready, and take full advantage of its splash damage—a single blow can usually wipe out several Combat Forms. The Flood don't carry weapons until later on in the level, so melee weapons are quite useful against them.
Scour the area for more Shotgun ammo—this is your main weapon during this mission, so always be on the look out for more cartridges. There are four Shotguns in the Storage Area, along with Battle Rifle ammo and the Assault Rifles dropped by your fallen allies, so make sure you have a full amount of ammo before moving on. There will also be a Shotgun to the left of the next room.
Inside this warehouse, engaging the Flood over open ground is clearly suicidal. Though only a few carry weapons, their sheer numbers and the scattered Pod infectors (reanimating additional corpses) make it easy to be overwhelmed. The recommended way of dealing with massive amount of Flood is to launch a brief attack before taking a defensive position and encouraging the Flood to come to you.
The doorway beside the metal stairs is the best place to make your stand. Generally, the Flood will attack individually or in small groups, which can all be easily dispatched. Larger packs of Combat Forms are more dangerous, but they can still be defeated with relative ease; just backpedal and fire point-blank Shotgun blasts at their chests. If there's a really big group, that's your cue to break out the Gravity Hammer.
There is some potentially useful equipment here, including a Trip Mine, a Bubble Shield, and a Regenerator; these items are common throughout the mission. The most useful is probably the Regenerator, as armed Combat Forms can rapidly deplete your shields.
The next wave of Flood is smaller. If a large group is headed for your position, shoot the Fusion Coils as they pass by them; the explosion should engulf a few. Your biggest concern is to avoid being surrounded. Otherwise, this battle should be relatively easy. The metal walkway that leads to the next room is home to a Flamethrower. It's enjoyable to use, but relatively slow and limited in range—which can be problematic, as you'll really need to destroy Combat Forms quickly on Legendary. Use it to torch the prone Flood bodies, which is quicker than using melee attacks.
In the next room, you'll find a weapons locker with Battle Rifles in it; it may be a good idea to swap your Shotgun for one of them. After that, as you walk toward the next door, you'll have a Cortana Moment. Pull out your Gravity Hammer here. If you don't have your hammer then use the trip mine. When you reach the twisting metal corridor, be ready to deal with a large group of armed Combat Forms. If you sprint forward, you can take them all out with two well-aimed strikes. Continue past the Suicidal Marine, exit the warehouse, and you'll arrive at Lakebed B.
Shadow of Intent (Rally Point Alpha)
A friendly Phantom will deploy a squad of Elites at your location. They are a big help, but they can be a nuisance if you're actively seeking a high score—valuable kills will be stolen. The Elites also tend to jump into the line of fire when you least expect it, and their furious grenade lobbing could lead to you being unintentionally betrayed. To increase their lifespan and your score, skip the cutscene and snipe the attacking Flood with your Battle Rifle as you approach.
(Occasionally, the Catch skull may cause them to grenade themselves off the cliff and to their deaths.)
The Elite landing party will thin down the force of Combat Forms situated on the side of the lakebed. If you find any intact Flood bodies, destroy them to prevent their reanimation. When the area is visually clear, search for two Plasma Rifles to dual-wield. Approach the ramp slowly, and immediately pull back into cover as Combat Forms leap from the cliff to your left. Dispatch them at range using your Plasma Rifles. They usually carry Brute Shots and Shotguns at this point, so take care not to get hit, and be ready to duck behind cover.
More Combat Forms will advance down the ramp, so knock them over before they open fire. Two Carrier Forms will waddle down the slope; toss some Frag Grenades or Plasma Grenades to rapidly blow them up, and then aim to mop up the swarm of Pod infectors as they approach. Make sure no nearby corpses are reanimated.
Get More Points
To get more points, lower the Elites' shields while fighting the Flood. They'll die, but most times that will draw their Swords and soften up the Flood. These Elites will focus on the Pod infectors, for unknown reasons. Once they die, collect the Swords. Make sure that the sword that you've collected after destroying the Flood has 85% or more energy.
Use a Plasma Grenade to completely destroy the Flood.
As you enter the factory, avoid trying to fight the Combat Forms in the open. Stay near the damaged truck and try to draw the Flood out. Shoot each of them one-by-one as they approach.
As you leave the low corridor, additional Combat Forms will drop down from the ceiling. They'll generally land in close proximity of each other, so drop your second Plasma Rifle and throw a grenade or two into their midst. Then, retrieve your second Plasma Rifle and dispatch any that remain. Quickly run underneath the walkway to avoid being shot by the second wave. They'll arrive through the door above, so stay alert. Wait at the bottom of the stairway to take out any that run down. The Arbiter will take care of the rest. It could help to deploy a Regenerator at the bottom of staircase.
On the upper level of this building, you can find a second Flamethrower. This isn't the best of weapons to deal with the Combat Forms, but it should make easy work of the Pure Forms outside.
Another group of Elites is usually (but not always) dropped off as you exit the warehouse—they will be deployed if the previous group of Elites has been wiped out. This time, take advantage of their aid as you fight the initial wave of Combat Forms and the Pure Forms further ahead. Be warned that they will take little consideration when you run in front of them—it's easy to get caught in the crossfire and get killed by one of their Plasma Grenades.
Make sure the immediate area is clear before picking up an Energy Sword to replace your Plasma Rifles. If you can't find one, pick one up the next battle. Detach the Machine Gun Turret and carefully walk along the concrete path. Use short bursts of turret fire to temporarily disable the Ranged Forms that attack from distance as you push forward. If you rush, there is a big chance of being surrounded, so slowly advance, dealing with any Combat Forms that draw near, and choose cover that shelters you from incoming projectiles.
At the far end of the concrete path, use grenades to weaken the large groups of Flood before using your Energy Sword to finish them off. With other weapons, it would take forever to kill a Tank Form on Legendary, but the Energy Sword can down a Tank Form in one swipe. Move into the corner of the area and lure the Flood to your position. They'll come in manageable numbers, so they shouldn't be too difficult.
Infinite Devil Machine
Finish your objective by heading into the Flood ship and retrieving what seems to be Cortana. Remember not to fire your weapon—if a stray bullet hits a Flood Blister growing on the wall, it will release several Pod infectors, and since killing them does not earn you any points, there is no benefit in fighting them.
This level is relatively easy, due to its short length and the aid of your allies through out the level. Although the Elites may be annoying you by Grenades or other things, never think of betraying them. It will cause the Arbiter to slash you with an Energy Sword and also points subtraction. To conclude, use cover and aim better will get you over easily.
It Followed Me Home... (Mission Start)
Backtrack along the path you took to get to the Anti-Air Battery. On the left side, when first heading south, there is a Gravity Hammer lying behind an overturned vehicle. Swap your Assault Rifle for it.
As you enter the next section, where Marines are waiting for you, Combat Forms will drop from the roof. Take advantage of the Gravity Hammer's splash damage or your Battle Rifle's headshot capability to make short work of the Flood. After the engagement, you may swap your Hammer for a Shotgun if you so desire. Continue through the office behind the door through which you entered this area on The Storm.
Here, you will see a Marine being ambushed by Pod infectors. Your chance of saving his life is slim, though you will have to kill the Pod infectors regardless. Inside the main room, there is a swarm of Combat Forms. Get their attention by shooting or lobbing frags at them, and then pick them off with a Shotgun or Battle Rifle as they climb (or jump) up the stairs to the office. If you still have your Hammer and are feeling daring, you can exit the office, smash a few Combat Forms, and then retreat back to the office; staying outside, however, will get you surrounded and killed.
When the Flood have been dealt with, grab a Trip Mine, a Regenerator, or a Bubble Shield, and then continue forward. The next section after the wall of shipping containers contains additional Combat Forms; kill them. Go up the stairs, across the bridge, and around. Drop through the hole for a Cortana Moment. Pass the two weapons lockers, and then turn right. There are several Flood camping around the 180° turn; counter this by throwing two grenades, and then finish off any survivors. (Frags can be bounced off of the wall, to hit them without exposing yourself.) Pass the Suicidal Marine and continue.
Shadow of Intent (Rally Point Alpha)
Several Elites will be dropped at your location. Aid them in killing the Flood, and then continue. Go through the indoor area. A swarm of Flood will jump down from where a swarm of Drones flew in The Storm. As always, you will be fine if you don't let them surround you.
When you get outside, kill the Stalker Form. If it turns its back and changes form, it can be assassinated. Kill the Combat Forms in this area, then deal with the Pure Forms. Grab the Machine Gun Turret, detach it, and walk down the path to the right, killing anything in your way. When you pass under the metal pipe, be careful because there are several Pure Forms. Kill them and make your way to the entrance to the Flood ship.
Alternatively, if you are skilled at deploying equipment in precise locations, you can place a Deployable Cover in front of the Machine Gun Turret (without detaching it) to create an instant machine-gun nest that is extremely powerful against Pure Forms. Like any machine-gun nest, it is weak against flanking; keep your Elites alive (give supporting machine gun fire) so that they can defend your flanks. If you are playing Co-op, your partners can defend you. One Ranged Form will be out of the machine gun's range, so kill it with your Battle Rifle. Make your way to the entrance to the Flood ship.
Infinite Devil Machine
There are no enemies here and no risk of getting lost, so just charge through until you find Cortana. Don't shoot the Flood Blisters unless you want to make the journey harder for yourself. A swarm of Pod infectors can overwhelm you, especially on higher difficulties like Legendary.