Floodgate is the fifth playable level in Halo 3. See Floodgate (Halo Nation) for general information. Due to the lack of danger from enemies, there will be only one tutorial for this level, though difficulty-specific tactics and notable details will still be included.
This level centres around quality movement and good box and DC launching technique.
It Followed Me Home
The beginning of this level focuses on pure movement. Do slide jumps down the hill and make your way to the corner. A boost similar to an over-jump can be obtained off of the overturned barrier at the top of the ramp. To perform the first box launch, round the corner and throw a grenade at the base of the two boxes on the right. Pick up the gravity hammer and then shoot one of the boxes forward so that it is beyond the confines of the hallway. Do the box launch and head on into the factory.
Pick up the spike grenades and throw one spike grenade behind the weapon crate at the point where the floor tile corners intersect. This will rotate the weapon crate and give you the perfect orientation for launching. Launch off it and continue on through the factory. You should pick up a DC from the small bridge just before entering the Cortana zone. grenade jump or hammer jump up left onto the upper platforms. Perform the Cortana skip and then jump up above the Maze room and box launch to the hallway leading to the outside. Melee or jump to delay a checkpoint here, as you want a checkpoint to happen immediately after the next cut-scene finishes playing.
A cortana moment will activate if you fall into a trigger volume in this room. The trigger volume only extends to about head-height above the floor, so as long as you stay above that height you won't trigger the cortana moment. The most common method to perform this skip is to jump onto the closest weapon crate on the left side of the room, then bank a frag grenade off the white label on the other weapon crate and jump so that you move the crate and land on it by the door. You can then crouch and walk forward and off the crate to skip the cortana moment. There are other ways to do this skip and it is possible to do it without any grenades or hammer swings.
Shadow of Intent
If you delayed your checkpoint, you should get one as soon as the cutscene is over. Look to the left of the crane and perform a DC launch towards the other end of the lakebed. This launch is relatively easy on MCC, but much harder to do well on OG halo 3. If your dc launch falls short (or you decide to walk), you may trigger a wave of enemies to spawn in at the doorway to the next area. On easy this is not a problem. On Legendary it is important to have you BR out and kill as many flood as you can while bee-lining for the doorway.
This is the one section of this level where Legendary and Easy strats may differ. On easy, to perform the hallway box launch you must either destroy the large box with a spike grenade or hit it with a hammer swing to weaken it, and the second hammer swing will break the box and launch you. While this method is also viable on Legendary, getting a poor launch will likely result in your death. On Legendary, the safe strat is to keep the BR out and run along the left-hand corridor of the building, killing flood along the way.
The Flood, It Has Evolved
Box launch or walk out into the large open lakebed. There are literally an infinite number of strats for this area, but the most common and easiest method is to collect the DC by the large crates, hammer jump up onto the first crate, and DC launch accross the lakebed to the crashed ship. This is slightly easier on OG halo 3, as the more powerful MCC launches can kill you. An alternative method is to collect the tripmine by the turret, angle one of the small boxes near the edge of the platform, and perform a tripmine assisted Box launch (OG H3), or box launch normally then trip mine jump in mid-air to make it accross the gap (MCC).
Infinite Devil Machine
Make you way through the ship and optimize movement just like at the beginning of the level. Go up high on the left hand side before entering the control room and perform a gravity hammer assisted slide jump to reach the console early before the gravemind moment finishes.