Note: The Checkpoints in the walkthrough will appear to be slightly out of order. This is not actually the case. I am unsure if you can actually do the first two towers in the opposite order from my guide but I AM sure that this is how they are named in the game and that this is the order in which I did them. Each tower has its own rally point.
Rally Point Alpha
You'll start still aboard the UNSC Infinity. Climb aboard the Pelican that Lasky set up for you. After an inspection, you'll find yourself in front of a launch tube. Hit the thrusters and take off. Cortana will set a waypoint for you once you're clear of the ship. Circling the ship will be a bunch of Phantoms. You can use the primary weapon but if you switch, you'll bring up a Spartan Laser with infinite ammo and a quick recharge. I preferred this when fighting the Phantoms. You could keep your distance and take them out with just a couple of easy shots.
Rally Point Charlie
When the landing platform is clear, fly over to it and touch down. Along the hull of your Pelican should be a pretty good assortment of weapons so take your pick. I found the SAW to be pretty useful but take whatever ever you like and head inside. The building will quickly open up into a large room and there will be a couple of Watchers hovering around in front of you. When you eliminate them, be sure to watch the left and right branches because there will be more Watchers there.
Head straight ahead to the first switch and flip that. There will be a couple of weapons here if you need them including Lightrifles and Boltshots. After you've flipped the switch, you'll be attacked by a few Crawlers and Watchers. Use the Lightrifles to take out as many enemies as you can. While you're doing this, Cortana will mark the first waypoint. With the enemies thinned, it should be much easier for you to get to the switches.
Start making your way to one of the arms and move towards the switch. There's a lot of cover on the bridges so take your time moving up. You'll encounter only a single Knight but there are a LOT of enemies and you can become overwhelmed easily if you rush. When you finally get to the waypoint, destroy the power source.
Again, you should be able to find more ammo around here and you'll almost instantly be attacked by more enemies. Like before, you've got a pretty good position with plenty of cover so start taking out enemies. Cortana will mark the next power source but your enemies won't just attack from there. From your left will be a bunch of Crawlers and maybe Watchers. Directly ahead of you will be a dense field of enemies including a couple of Knights.
Go down the ramp in front of you and stop at the midsection there. There may be enemies here including Knights so take those enemies out first and then turn towards your objective. Get rid of enemies out front and then you can approach but be careful because there may be an enemy or two in the structure (again, likely Knights since they hang near the back).
Destroy this power source and then turn right to make your way to the last one. You'll encounter a couple of Knights and Watchers along with one of their large Turrets. Try and take that out first since it has no shields, no Watchers, but can pin you down. If you need a better weapon, there are some Lightrifles in the structure you just left.
You probably have enough ammo to take out most of the enemies from range. Make your way up over the large hump and down onto the bridge beyond. Turn left and you'll find a lot of Knights and Crawlers below. At this point, you may be running low on ammo so try to draw some Crawlers up to see what you can get for ammo. You may have to make your way up between cover to close the gap depending on what you have left. Watch out for Watchers behind the structure on the right - sometimes they can get stuck behind something but still protect their allies.
With the enemies destroyed, get rid of the power source and then make your way around the rest of the outer ring towards the exit. About a quarter of the way around, follow the ramp up to the middle platform and exit that way but whatever you do, be careful of enemies. There won't be a lot but they are going to be the tougher varieties.
As you exit the structure, you're going to come across a simple Knight but just behind him will be a Promethean Battlewagon. The Battlewagon moves similarly to a normal Knight but carries a lot more firepower. His primary weapon is going to be the Incinerator Cannon which is easily the most powerful weapon in the game. It shots a shot with four little bomblets that explode a split second after the primary shell explodes. Each one has about the power of a Rocket.
Be careful and stay behind cover. You need to move only after he's shot. Doing so, you should be able to get close enough to deal massive damage quickly and kill him. When he's dead, head back outside and hop into the Pelican again (after restocking on ammo).
Rally Point Bravo
Back in the Pelican, Cortana will set a new waypoint for you. When you get close to it, you're going to come across more Phantoms swarming around the tower. Take these out the way you did with the last group and then make your way in to land as you did before.
Inside the tower, you're going to come across a sort of tram system. You need to use this to cross the giant gap and get to your destination on the far side. As you slowly make your way across, you're going to be attacked by enemies on the sides. They'll primarily be stationed on giant structures on the right and left sides. When your tram gets to those points, you'll get stuck and have to raid these structures in order to reactivate the tram. If you need ammo at any point, there are weapons and grenades on the bottom most level of the tram.
As you start moving across, keep your eyes to the right and you'll find the enemies waiting for you. There are quite a few Sniper Jackals on this structure so it is best to take out as many as possible as you approach. When you get stuck, cross the bridge and you'll start engaging Jackals and Grunts on the lower levels. Fight your way up to the top and you'll find some Elites as well. Grab a Plasma Pistol off of a dead Grunt and use it to take out the Elites' shields.
When you've cleared the top level, you'll find the switch dead center. Along the back wall near the switch will be more ammo. Take what you can and start making your way back to the tram. Take your time because the tram isn't going to leave until the enemies on it have been cleared. On the next section of the structure, you're going to come across a couple of Knights, one of which is a Battlewagon. Go slow and give them the one-two plasma punch. Not far beyond them will likely be a Jackal which won't be too dangerous but could be quite a surprise.
On the tram below will be a whole host of Knights and Crawlers. You should also see some Crawlers walking around on the walls in the distance. Try to take out as many Crawlers as you can possibly find. When they are all dead, waste the ammo on your long range weapon to take out a Knight or two before going down. When you've sufficiently thinned their numbers, you can jump to the tram below, clean up the remaining enemies and continue your journey by hitting the control switch at the top.
With the tram back in motion, remember to check out the rest of the tram for ammo. Be careful because soon a couple of Knights will ambush you. Try to find a spot with good cover and pick off the Knights one at a time. Try to be aggressive if you can because on the left side you'll start coming up on another structure.
Instead of Covenant, you're going to just have a whole bunch of Prometheans on your hands. Try to pick off as many Crawlers and Watchers as you can from the ship but be careful because the Crawlers will likely attack eventually. Start working your way up the tower and at each level you should keep an eye for ammo caches to help you. It will be primarily Crawlers as you work your way up but towards the top you will encounter a large group of Knights as well. At least one of them will be a Lancer and will hit you from long range so try and keep something between you and him while you work on the others.
Be sure to watch for Auto-Turrets up here as well. If you can push forward slowly, you should keep the Lancer travelling back and you might be able to get to the switch without killing the Lancer. This can be used as cover to push further ahead until he runs out of room. Kill him and then get to the edge of the platform. Your tram has been hijacked like it was last time. Deal with it just as you did last time by using long ranged weapons.
Move back down to the tram when it is safe and wait for it to reach the final destination. Grab ammo and then head to the waypoint. Walk in and it should deactivate it. As soon as you do, you'll be attacked by a bunch of Watchers. Get back on the tram as quickly as possible. Hit the switch to get the tram moving and then head to the lower decks. From here, it will be safer to deal with the Watchers because the levels above will provide cover. You should be able to take them out one at a time from this position.
If you get rid of them all before you get back to the beginning, you just get to wait. Stock up on ammo and then head back to the Pelican. Cortana will set another waypoint but there shouldn't be any more enemies here so you can just fly in and set down. Grab whatever weapons you like and head into the building.
Rally Point Delta
Inside the building, ride the lift up. At the top, be careful where you go because the floors will start moving around. Jump gaps as appropriate and when you get over the bridges, you'll find some towers with Covenant all over them. Kill the first set of Grunts around the first tower. The path will split and lead to two other towers. Each of these towers will have a Turret on it.
Take out each of the Turrets and then pick whichever path you'd like to take. The next tower you move to will have Jackals on it. The tower beyond will have some Elites and be connected to the far tower that you didn't take so be careful as you take this next tower to not be too hasty. Some of the Elites will also have Fuel Rod Cannons. Take them out however you see fit (there's plenty of needlers around here) and take the Fuel Rod Cannons. This should help you with the final tower. This tower has a lift on it and a couple of Hunters will come down. Use the Fuel Rod Cannons and Grenades to make short work of them.
Before taking the lift, there's ammo on either side of it. Stock up and head back to the previous tower. Turn right and go to that tower. If you go around the far side of the tower and look towards the tower, you should find a Terminal. Catch the lift when you are ready.
At the top of the lift, you have two options. The fast and dangerous method (may take a couple of tries) has you grabbing the Banshee on the first tower, flying under the tower platforms for cover and then jumping out of the Banshee by the exit. The slow method has you working through the multiple towers towards the exit. This is slower and more steady but will take some time as the towers are dense with enemies and heavy fire power. The Banshee method can work at least as high as Heroic. You'll need to pull down under the platforms and back up to fly the Banshee in between levels by the exit.
Slip through the door. Activate the console and jump through the window.
