Pelican Down

Before heading to the West AA gun, collect two items in the marshy land near the Pelican: the first Spartan Core on a ruined ship, the first UNSC audio log nearby, and a second UNSC audio log in another ruined ship.

Head towards the western AA gun until you reach the mechanical elevator. Turn left before going up to find a second Spartan Core.

Use the elevator to reach the top and defeat enemies. Free any imprisoned Marines for assistance. Find the first Banished audio log near the freed Marines.

Enter the West AA gun's control room using the gravity lift beneath it. Interact with the terminal, triggering a cutscene (a non-interactive cinematic the game plays automatically) with Escharum. Shoot the exposed cooling pylon to destroy the AA gun. Exit the control room before it explodes. This unlocks the 'One Down…' achievement.

Before leaving the West AA gun area, find the Mjolnir Armor Locker on a nearby hill. This provides the Warmaster's Prize Battle Rifle Coating.

Make your way to the East AA gun. Along the way, find a UNSC audio log in the ruins of a ship in the marshy land.

At the East AA gun, defeat the two Hunters guarding a terminal beneath the gun. The Gravity Hammer can be found in the cave behind the terminal. Use it by spamming melee attacks on both Hunters if they are close together to stun and damage them quickly. If not, focus on one at a time, using cover (press the cover button to crouch behind walls or objects) to regenerate shields if needed. Alternatively, use other power weapons from a distance, such as the Ravager, grenades, or blast coils.

After killing the Hunters, use the terminal to destroy the East AA gun. You will be contacted by Hyperius.

After destroying the East AA gun, obtain the nearby collectibles.