Part 1: A Whisper in a StormEdit
The first objective you have (the Arbiter) is to locate this Heretic Leader who is thought to be taking refuge someplace on this installation. Armed with an Energy Sword and a Plasma Rifle, you are well-fitted for battle. Follow the rest of the group as they lead you down a ramp. Meet up with the rest of the group and an Elite will appear, attempting to crack the door lock. Head through and another door will be opened.
The storm has masked your approach, so your opponents aren't expecting any attacks just yet. Use this to your advantage and activate your active camouflage with the White Button just before entering. A few Elites and Grunts are patrolling the area. Take advantage of your invisible state and use the Energy Sword to give this group a wake-up call. With that, some shots will begin shooting and the action begins. Hoorah.
There are a few Elites around using Covenant carbines rather than Plasma rifles. Use this to distinguish between friend and foe... not to mention that those Heretics are wearing ugly headpieces anyway. Use the Sword or Plasma rifle to take down the forces in this first area. A ramp to the right leads up a small slope leading downward. Some more enemies, including two or three Elites are also located here.
Again, use your active camouflage to hopefully sneak behind another enemy. Be wary of the blue paths on the ground. Stepping on them will cause you to slowly move down the slope. It can be hard to notice as you move along this blue path. Just don't let yourself fall off the edge at the other end. Anyhow, once you clear this area, make your way to the other end of the area and take either path on the left or right to head downward to the next platform. Don't fall off the edge by the way.
Probably as of now, the battle has sort of died down and silence is flooding back in. The next room has a fully charged Plasma rifle on the ground that can be picked up. For the next room, dual-wielding Plasma rifles are strongly suggested. At first, some Grunts will appear here and there. However, an elevator on the other side will send up a group of two Elites and a bunch of Grunts. Like I said, dual-wielding Plasma rifles can quickly quell the Elite threat. Once they are down, the Grunts will fall like dominoes.
Anyhow, this room is a dead-end. The only way forward is down the elevator. If your allies are still alive, lead them onto the elevator that the enemy group came up on and head downward. After about halfway down, you should notice some Sentinels holding blue containers floating around. Don't shoot until you reach the bottom to avoid letting your opponents know that you're here.
Now at the bottom, begin shooting like mad at the various Sentinels floating around. Plasma weaponry works best against them. Take care of those Sentinels as quickly as possible. Some Elites will begin flooding in through the doorways on the upper level. While fighting those floating scraps of metal, don't step into their beam for too long. It can wreak havoc against your sub-par shield system.
Your commanding officer at this point should order you to open the hangar door. The faster you get to that, the faster you will receive reinforcements so get over there as quickly as possible. The hangar door is the big window at the opposite end of the elevator. You can find the activation panel on either side of the door. Just press the X Button and the Phantom will fly in to save the day. Let it blast away at the enemies hanging around and take full advantage of the reinforcements it drops.
Once the Phantom flies out, begin to mop up the remaining forces. Once again, enemy forces will scramble in through the upper pathways. If your shields are down, take cover while your allies attack the Heretic platoons. If possible, use the blue containers around the area as grenades. Shoot at them and they will explode hopefully on your enemy's side. Be wary of Grunts wielding Fuel Rod Guns around the area. They act like miniature Rocket launchers.
It should take quite a while before you can clear the room. If possible, keep as many of your allies alive as possible. The more you still have, the better. You will know when the Heretics in this room have been totally defeated when an Elite wielding an Energy Sword appears from a door just one level below the area where the elevator dropped you off. Don't stand too close (obviously) and kill off the final Elite. When the room is quiet, begin making your way through the doorway just below the elevator shaft. By now, it's probably open.
Let your allies follow you as you round the next corner. Another large group of Grunts and Elites are just a few feet ahead. Quickly throw a grenade into the group and jump back. The tight corridor can be used in your favor as it disallows your enemies to move around too much. Mop up the remaining Elites any way you choose. Once opposing forces in this sector are defeated, continue once again.
At the end, you will find an open door on your left. This medium-sized room is infested with enemy forces up and down. I'd suggest running to the top platform as there are less enemies up there. Use a liberal amount of grenades to scatter your enemy and then wipe them out with your mad skills. Use the upper platforms strategically and run up there when you need to recharge your shields.
Once you clear out the majority of the enemy forces here, head to the door at the other end of the room. It's pretty easy to find considering the size of this area. Anyhow, exit into the next corridor area and some Grunts will make their appearance. Put them in their place and advance. The next intersection will have Elites coming out through the right door and Grunts on the other side. Throw a quick grenade to scatter the Grunts and finish off the two Elites which also appear.
Now with that force down and out, have your team continue once again. Anyway, from this point on, it's all a matter of transcending the hallway down and down. Some more Grunts appear ahead of you. Watch out for the Elite that pops out the locked door in front of you. Round the next corner and some more Grunts will appear as if begging you to kill them. Grant their wish. Finally, you should reach a doorway. Before you enter, suggest engaging your active camouflage.
Some Grunts should be sleep in the little niche ahead of you. Give them a good smack in the head to take down both of them. As soon as the Sentinels notice you, start shooting them down like mad. From the entrance, continue left making your way through the piles of Grunts. Watch out for the Elites and do not let them overwhelm you. Again, use the blue containers hanging around as grenades by making them blow up nearby enemies.
It shouldn't be too difficult to make it toward the end of this area. However, if you aren't careful, prepare to be overwhelmed. There are a bunch of niches and areas where you can hide for cover. When ready, go through the other door at the end of the room. Just as you probably expected, more Covenant. Whee...! Once that team is cleared out, head through another doorway and you should notice some Banshees ahead of you... through a window of course.
Some enemy Heretics are just outside. The two doors on your left and right are locked as of now. However, the one on the right (upper platform) will open to release another platoon of enemies. Once again, take your trusty Plasma Rifle and zip right through. Following, head through the door that the Heretics came out of...
Part 2: To the HuntEdit
Your commander will radio in again and tell you that the Heretic Leader is escaping. Objective now would be to follow him. With that, he offers to send one of his Phantoms to help clear the way for you. Judging by the looks of things, you will probably need it.
Exit through the door and find yourself outside in the raging storm. Two Banshees are parked just ahead. Wait for the Phantom to appear and get in one of them. Once you begin flying, the Phantom will begin to move. This part of the level requires you to follow. However, there will be abundant enemy Banshee patrols throughout the area. Right off the bat, a Banshee will fly in toward the Phantom's rear.
Use your cannons and Fuel Rod Gun to throw that enemy Banshee away. While the Phantom is slow, I'd suggest sticking near it anyway. It will help you destroy enemy Banshees as they come. Furthermore, you don't know where to go, so the Phantom will lead you. Anyhow, some cool music will pop in. Begin following the Phantom and defeat the various Banshees that attack your flank. Don't let your craft get too busted up at this point.
As you follow the Phantom, you will occasionally come across platforms in which Grunts with Fuel Rod Guns attack you. Considering the power of those cannons, you will really want to do your best to avoid their shots. Kill off the Fuel Rod Grunts as best you can. After the first wave of these, the Phantom will lead you to another part of the installation. While the Heretic Leader is not here, you might want to take advantage of the two Banshees parked on top of the platform. Your Banshee is probably a bit busted up at this point so take either of the new ones.
Continue to stay around the Phantom and it will lead you to another heavily guarded installation. This platform is also a dud and the Heretic Leader is nowhere in sight. Stay close to the Phantom and shoot off any enemy Banshees on its tail. The Phantom does bear heavy firepower, but it does have its blind spots. Destroy the Banshees and the Phantom will finally locate the supposed location of the Heretic Leader. Quickly begin destroying whatever is guarding the landing zone and get yourself on that platform. Lots of enemies are apparent so kill them off with caution.