The Swords of Sanghelios and the Fireteam stand up against the Covenant. You must gain the control over the Sunaion by clearing all the Covenants and find the Guardian. Let's check out how Locke with Fireteam and Swords of Sanghelios pair up and fight against the Covenants in Mission 12: Battle of Sunaion.
Once the Cutscene is over and you take control over Locke, the first thing you must do is locate and destroy the Air Defenses. Head straight you'll notice the First Air Defense so just go ahead and disable it. This will help the Havoc on incoming in Sunaion.
Note: Perform Melee on cannon's main battery to disable it or keep shooting with powerful guns like turrets.
According to Arbiter, there are more Air Defenses Cannon on the way towards the Guardian so we need to move forward and disable them all. So head straight towards the door to locate the next cannon and disable it. you need to take care of a bunch of enemies along with the cannon.
From Here you'll split up from the Arbiter and his soldiers, you keep moving forward and disable the turrets and deal with few more enemies. Make sure you keep a track on your Team for assistance. Use your D-pad and the scan to locate the next Cannon. Now move towards the Door which will bring you to the area where 3 cannons are located, take them down along with the foes.
Later look for behind the second cannon you'll notice the Elevator, use the elevator to take you to get to the Undercity. Keep scanning nad keep a check on your ammo if you're about to run out of one. If you are totally out of ammo then you can operate the nearby Shade Turret to hunt the enemies down and then pick up their ammos.
Head straight and you'll come across a Big fight against the pair of powerful hunters and bunch of Prometheans to stop you. Now locate the Guardian, use the scanner to locate the next elevator which will take you up to the Guardian. Keep following the Path until you reach the Guardian Statue rising and will pass on a shock wave.
Now that everything is breaking apart you need to run and escape the area. Avoid falling off the edge and as you jump across the broken bridge keep moving forward. On your way up you'll encounter few Crawlers so kill them and continue moving up until you see Arbiter showing up. Now get ready to face off some Prometheans in such situation where nothing is going right.
After defeating the Prometheans, Warden Eternal will show up and now you need to use your best strategy to take him down. Once he is about to get beaten he will disappear and a cutscene will start, end of Mission 12: Battle of Sunaion.