Finding your way around the Floodgate mission shouldn't be much of a problem. Since you're backtracking your footsteps all the way the Lake bed A of the previous mission. 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, like in all other Flood-only mission, 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. This is arguably the easiest level with Iron activated. Simply because it is very short.
Storage Area
Make your way down the hill, and enter the Storage Area where you fought the Chieftain. There will be a building on your right. Just after the checkpoint, a Flood combat form will be on the roof and jump over the street. If you kill this Flood during his jump, he will drop the Fog skull. Collect grenades, a Gravity Hammer and a Shotgun en route to your first encounter with the Flood. Combat Forms will drop down and start attacking your allies. The safest choice would be to hang back and wait for them to approach you. Jumping in to save the Marines is a risky decision. Seeing as a single Infection Form can make easy work of one, their chances of survival are close to nil. Make sure you have your Gravity Hammer at the ready, taking full advantage of its wide area of effect. You could usually wipe out several Combat Forms headed towards you with a single blow. The majority don't carry weapons until later on in the level, so melee weapons are a good choice.
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, 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.
Warehouse
Inside this Warehouse, engaging the Flood over open ground is clearly suicidal. Though only a few carry weapons, their sheer numbers and scattered Infection Forms (reanimating additional corpses) make it easy to be overwhelmed. The recommended way of dealing with massive amount of Flood is to launch an initial 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. They shouldn't be that difficult to deal with. Larger packs of Combat Forms are more dangerous, just backpedal and fire point blank Shotgun blasts at their chests. If it's a really big group, that's your cue to break out with the Gravity Hammer.
There are some potentially useful equipment here, including a Trip Mine, Bubble Shield and Regenerator. You'll find that these items are common throughout the mission. The most useful is probably the Regenerator. Combat Forms carrying weapons could lower your shield rapidly, so it's useful to have something which can counter that.
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 would be 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 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.
When you reach the twisting metal corridor, pull out your Gravity Hammer and be ready as you approach the first corner. A large group of Combat Forms have been impatiently awaiting your arrival, and now their main aim is to ambush you. If you sprint forward, you can take them all out with two well-aimed strikes. Exit the warehouse and you'll arrive at Lake bed B.
Lake Bed B
A friendly Phantom will deploy a squad of Elites at your location. They are a big help, but they could also 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.
Still, the Elite landing party will thin down the force of Combat Forms situated on the side of the lake bed. If you find any intact Flood bodies, destroy them to avoid their reanimation. When all is visually clear, search the area for two Plasma Rifles to duel 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.
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, them aim to pop the swarm of Infection Forms as they approach. make sure no nearby corpses are reanimated.
Whilst you may grieve over the loss of your Gravity Hammer, the Plasma Rifles are exceptionally more useful during the following battles.
Factory B
As you enter the factory, avoid trying to fight the Combat Forms in the open. Stay near the damaged truck and try to gain the Flood's attention. Shoot each of them one-by-one as they approach.
As you leave the low corridor, further Combat Forms will drop down from the ceiling. They'll generally land in close proximity of each other, so 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. Thel 'Vadam will take care of the rest. It could help to deploy a Regenerator at the bottom of staircase to your advantage.
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 they should make easy work of the Pure Forms outside.
Lake Bed A
Another group of Elites is usually (but not always) dropped off as you exit the warehouse. 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 in place of your Plasma Rifles. If you can't find one, pick it one up the next battle. Grab the Machine Gun Turret, then 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. Usually it would take forever to kill a Tank Form of Legendary, with a single Energy Sword slash is enough to take one down. Move into the corner of the area and lure the Flood to your position. They'd come in manageable numbers, so they shouldn't be too difficult.
Finish your objective by heading into the Flood ship and retrieving what seems to be Cortana.
Debriefing
New Enemies: Infection Form, Carrier Form, Combat Form, Stalker Form, Tank Form, Ranged Form
New Vehicles: --
New Items: --
New Weapons: Flamethrower, Energy Sword
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. And also remember that Tilt isn't recommended during Flood-based missions. You can freely follow the weapon instructions during this walkthrough because you'll receive new ones at the start of the next mission (which are the Assault Rifle and Sniper Rifle). 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 billion other techniques that work just as well.
Exterminator Achievement Milestone: 15,000 points
Recommended Multiplier: 6+ (Tough Luck (1.5) + Catch (1.5) + Black Eye (1.5) + Iron (3) + Tilt (2) + Mythic (2))
TIME BONUS
0-10 min - 3x
10-15 min - 2.5x
15-20 min - 2x
20-25 min - 1.5x
Suicidal Marine
Shortly after you get the Flamethrower, there will be a Marine in the corner talking to himself about his squad's infection by the Flood, and how he just "did them a favor." This is odd, because the Marine bodies around him show no sign of flood infection. After this, he says something about how he should do himself a favor too, and points his pistol at his head, seemingly about to kill himself, although he alternates between this and putting his gun down and shaking. It should also be noted that he sometimes will not have his gun out, and instead be pointing his empty hand at his head. The suicidal marine has never been observed to actually kill himself, but many players do, either out of a desire to end his misery or for more sadistic reasons. A glitch has been observed that if you melee him he may get up and help you fight.
Walkthrough
It Followed Me Home . . .
Backtrack along the path you took to get to the Anti-Air Battery. On the left side when first heading south, behind an overturned vehicle, there is a Gravity Hammer. Swap your Assault Rifle for it. Go to the area where the Marines are and Combat Forms will drop from the roof. Take advantage of the Hammer's area of effect or shoot the Combat Forms in the tentacles. Swap your Hammer for a Shotgun, if you would like. Continue through the office behind the door through which you entered this area on The Storm.
There will be a Marine and several Infection Forms. Your chance of saving the Marine is slim, but kill the Infection Forms. Inside the main room, there is a swarm of Combat Forms. Aggro them, killing a few in the process (use a grenade, perhaps), and pick them off as they climb the stairs (or jump up) to the office. If you are feeling daring, you can smash a few with a Grav Hammer, then return to the office. But if you stay down there, you will be surrounded and killed quickly. When they have all been killed, grab a Trip Mine, a Regenerator, or a Bubble Shield, then continue. The next room contains some more 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 boxes, then turn right. There are several Flood camping around the 180o turn (similarly to the infamous multiplayer strategy), so toss two grenades, then kill them. Pass the Suicidal Marine and continue.
Shadow of Intent
Several Elites will be dropped at your location. Aid them in killing the Flood, then continue. Go through the indoor area. A swarm of Flood will jump down from where a swarm of Drones flew on 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 AIE-486H Heavy Machine Gun 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 HMG for 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 range of the machine gun, so kill it with a 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 the terminal. Don't shoot the Flood growth pods unless you want to make the journey harder for yourself. A swarm of infection forms can overwhelm you, especially on the higher difficulties such as Heroic or Legendary.