The Arbiter Walkthrough
From Halopedia, the Halo wiki
Part 01: A Whisper in the Storm
Once you gain control, you'll see two Elites head down a ramp in front of you. Follow them down to find the rest of the Lance. As one of the Elites works at a locked door hand off your plasma rifle to one of the grunts. He'll be far more effective with it.
Head through the doors as they open, but, as you go through the second one, be sure to activate your active camouflage. Cut around the right side of the central pillar and assassinate the Elite. Grab the carbine, then use it to beat the sleeping Heretic Grunts to death. Be extremely cautious not to awaken any of the Grunts before you melee them, or they could wake everyone up, leading to your death. Head up the slope on the right side, noting the patrolling Heretic Elite, activating your camo as you go. Cutting around his back, assassinate him too, grabbing ammo for the carbine on the other side.
There is a Heretic Minor, the one in brown, a Heretic Major, the one in gold armour, and six Heretic Grunts in the area—two Lances, that is. The Grunts should be killed first, as their Needlers pose more threat than do the Elites. If you kill these Lances quickly enough, four more Grunts will join the fray, but they should fall to your Carbine.
Anyway, once you head down to the elevator, you'll meet four Heretic Grunts, two on each side of the elevator, who will soon be joined by a Heretic Lance from the elevator. It's possible to use your camo to get around to the elevator side and stick the Elite leading the Lance before he even reaches the top, but keep in mind that you'll then be vulnerable to fire from the Grunts. Your best option is to toss a few grenades while they are still stuck in the closed space.
This said, you may find it more efficient (depending on difficulty level) to take out the Grunts while your Special Ops team fights the lance coming via elevator while flanking them for an attack from the rear.
Get to the elevator and activate it. Down here is a hangar. A Heretic Elite is patrolling around your position; camouflage just before the elevator doors open and assassinate him. Then, head around to the left side walkway. As you approach one of the doors, a Lance will come out. Throw a plasma grenade to disorient them, then use your sword to finish them off. A Sentinel will descend in front of you and open fire. For now, though, focus on sticking the Heretic Major who will come up the ramp in front of you. Then shoot down the Sentinel with the Carbine (he should go down in just a few shots), camouflage, and open the hangar door with the switch at the bottom of the ramp. Another Heretic Elite may show up to threaten you, but you should quickly dispatch him with an energy sword strike..
After the Phantom shows up, Grunts with fuel rod cannons will show up and open fire on it. Kill the one nearest to you quickly, drop your Carbine for it, and jump onto the Seraph, then onto the other side of the hangar. Two Elites and two Grunts will show up from these doors; spam some fuel rods on them and clean up with the sword. Another two or three Grunts will probably show up from the doors closest to the elevator. As soon as they die, some more Grunts (two of whom have turrets) will show up from the door on the lower level. Two Elites, one with a sword, will follow them. Kill them before they have a chance to inflict devastating casualties on your allies.
After the battle, hopefully you will have some Elites left because you will want them. Grab a new energy sword and drop your empty fuel rod cannon for a plasma pistol; there's probably one lying around somewhere. If not, go with the carbine.
As you head down the hall, two Grunts and two Elites will show up. A recommended tactic is to use the overcharge of the plasma pistol on the Elites, then stick them with plasma grenades. You probably should take out one of the two Elites that show up soon after by doing the overcharge-stick tactic as well, but the other is probably attacking you by this time, so sword-lunge him with extreme prejudice. Beware, however, that having the sword and plasma pistols will leave you exposed to the two Grunts, who will easily kill you if you simply melee. That said, simply wait behind cover till your shields recharge and take them out with melees (from the plasma pistol; don't waste the sword's ammo). Then swap out the plasma pistol for a carbine and let your allies stealthily sneak into the next room.
Once they've moved forward a bit, move around them and headshot the two Grunts who are patrolling near your position. Then, camouflage, head up the ramp and cut around behind the two Elites who just showed up. Assassinate them both, as they're far too busy with your team to notice you.
In this area, you'll find two Elites already in the area, two more who join them a bit later, and about six Needler-Grunts. Stick all the Elites you can find, and take out the Grunts.
As you head down from this room, an oblivious Elite will patrol by you. Wait for him to pass you, then melee him from behind. There are two sleeping Grunts off to your left, so give them the same fate. Head down the hall, watching your motion tracker. There's yet another patrolling Elite, whose back is often towards you. Beat him down the ramp with a melee to the back.
This next area is a bit tricky, as you have to open fire, but an Elite flanks you as soon as you do. So, instead of taking the ramp down right away, keep going straight until you come to a locked door. Shoot it with the carbine, then switch to the sword. A very surprised Elite is behind the door. Cut him down to size with the sword, then turn around. A host of Grunts have heard the commotion and are coming up the ramp to check it out. Bizarrely, they will fire at your allies, not you. Shoot them all, or, more effectively, sticky one of them while they're distracted, and then move on down.
There's a weapon cache in the next area, including an energy sword. Take the sword, but bear in mind that we can exploit the Infinite Energy Sword Glitch here. So, we want to have an opportunity to empty the sword of ammo.
The next area has about seven Grunts, two Sentinels and four Elites. Kill them all with a combination of plasma grenades, sword and carbine. Note that you don't want to empty the sword yet, so save some ammo for it. You still have two battles ahead where you'll seriously want it.
The next room has two Elites patrolling. If you make a sound, four Grunts will show up to aid them. One recommended tactic is that you camouflage, assassinate the backmost Elite, then stick the next one and run past him into the next room. For some strange reason, the Grunts almost never follow you in there, and if one of them does, he's usually alone.
You can see the Heretic Leader through the window in front of you; unfortunately, you can't kill him at this point. Although it is possible to glitch through the window and cut him down, it results in an inability to complete the level and ultimately your death. Jump up onto one of the ledges on this room's sides. Watch your motion tracker, as two Elites and three Grunts that come through one of the two doors after a few seconds (hopefully they will be on your end of the room). You know what to do at this point. (If you have an energy sword, this is a good time to reduce its remaining energy to take advantage of the Infinite Energy Sword Glitch) Once they are all dead head through the door they came through to the landing on the other side of the window.
There are two banshees waiting. Pick one.
Part 02: To the Hunt
Hop into a Banshee and take off. The new Banshee has several downgrades from the old one. The fuel rod gun is much weaker (as are all explosives in Halo 2) and it can no longer hover. On a more positive note, it is possible to do aerobatics to barrel roll and loop your way out of danger, or into a good firing position. Across from us are two Grunts on turrets. If you still have energy in your sword, boost across to them, climb out of the Banshee and unleash the sword on them. Not only does it get rid of sword charge, it also gives us access to a fuel rod cannon which you should trade for your Carbine. Once you've taken them out, hop back in the Banshee.
From now on, you can do whatever you want. More fun alternatives are trying to get out on top of the Phantom, or just exploring the massive station. Also, hidden on the bottom side of the station is the Grunt Birthday Party Skull, one of the few skulls which is actually useful. After you've finished exploring the area, there are many Fuel Rod grunts. After killing them, stack up on ammo, as you'll need it for the next level. Anyway, when you reach the last part, you can simply bombard the turret grunts and fuel rod grunts and heretic elites from a distance. After they're all dead, get out, and you have finished the level.